Nitride semiconductor light-emitting device, method of fabricating it, and semiconductor optical apparatus

ABSTRACT

A nitride semiconductor laser device has a nitride semiconductor substrate that includes a dislocation-concentrated region 102 and a wide low-dislocation region and that has the top surface thereof slanted at an angle in the range of 0.3° to 0.7° relative to the C plane and a nitride semiconductor layer laid on top thereof. The nitride semiconductor layer has a depression immediately above the dislocation-concentrated region, and has, in a region thereof other than the depression, a high-quality quantum well active layer with good flatness and without cracks, a layer that, as is grown, readily exhibits p-type conductivity, and a stripe-shaped laser light waveguide region. The laser light waveguide region is formed above the low-dislocation region. This helps realize a nitride semiconductor laser device that offers a longer life.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor light-emittingdevice such as a nitride semiconductor laser device, to a method offabrication thereof, and to a semiconductor optical device provided witha nitride semiconductor laser device as a light source.

2. Description of the Prior Art

There have been test-fabricated semiconductor laser devices that lase inan ultraviolet to visible region of the spectrum by using nitridesemiconductor materials as exemplified by GaN, AlN, InN, and compoundcrystals thereof. An example of such a semiconductor laser device isreported in Japanese Journal of Applied Physics, Vol. 39 (2000), pp.L647-650. According to this document, a nitride semiconductor laserdevice is formed on top of a GaN substrate in the following manner.First, on top of the GaN substrate, an SiO₂ mask pattern is formed thathas stripe-shaped openings formed periodically therein, and then,further on top thereof, a layered structure of a nitride semiconductoris formed that has stripe-shaped waveguides (ridge-stripe structures).

The substrate is reported to be fabricated through the followingprocedure. On primer GaN having a SiO₂ mask pattern formed thereon, theSiO₂ mask pattern having stripe-shaped openings formed periodicallytherein (with a period of 20 82 m), a 15 μm thick GaN layer is formed byMOCVD (metalorganic chemical vapor deposition) to produce a wafer with aflat surface. This is a technique called ELOG (epitaxially lateralovergrown), which exploits lateral growth to reduce defects. Further ontop, a 200 μm thick GaN layer is formed by common HVPE (hydride vaporphase epitaxy), and then the primer is removed. Now, the fabrication ofa GaN substrate is complete. A semiconductor laser as actually producedin this way was estimated to have a life of 15,000 hours at 60° C. andat 30 mW.

One disadvantage of the semiconductor laser device described in thedocument mentioned above is that the fabrication procedure thereofinvolves three sessions of crystal growth (primer growth, MOCVD growth,and HVPE growth), and is thus complicated, resulting in unsatisfactoryproductivity. Another disadvantage is an unsatisfactory laseroscillation life, in particular under high-temperature, high-outputconditions (for example, at 70° C. and at 60 mW).

As is the case in the example described in the document mentioned above,typically used as substrates are GaN substrates, which have thereforebeen researched intensively in many research institutions. However, todate, no semiconductor laser devices have been obtained that offersatisfactorily long lives, and therefore longer lives are now beingsought in semiconductor laser devices. It is known that the life of asemiconductor laser device depends heavily on the density of defects (inthe present specification, defects denote atomic vacancies, interstitialatoms, dislocations, and the like) inherent in a GaN substrate. However,substrates with low defect density are difficult to produce, albeit saidto be effective in achieving longer lives, and have thus been researchedeagerly.

For example, Japanese Patent Application Laid-Open No. 2000-223743discloses a method of producing a nitride semiconductor light-emittingdevice structure on the top surface of a GaN substrate which is slantedrelative to the C plane. This helps to reduce lattice defects in thenitride semiconductor layer formed on the GaN substrate and thereby toachieve a longer useful live.

However, it has been suggested that, with conventional nitridesemiconductor laser devices like the one disclosed in Japanese PatentApplication Laid-Open No. 2000-223743, it is impossible to obtainsatisfactorily long lives when they are subjected to high-output agingover a wide area (or the whole area) on the produced substrate.Moreover, no mention is made of variations in characteristics amongindividual devices obtained after separation into discrete chips.

As described above, using a nitride semiconductor substrate having alaser structure produced by a conventional crystal growth techniqueoften results in an unsatisfactory life, or in a lower yield rate due tovariations in the characteristics of chips.

SUMMARY OF THE INVENTION

In view of the problems mentioned above, it is an object of the presentinvention to provide a nitride semiconductor laser device that offers alonger life, to provide a simple method of fabrication thereof, and toprovide a semiconductor optical device provided therewith. It is anotherobject of the present invention to provide a nitride semiconductorlight-emitting device that can be fabricated with reduced defects and atan increased yield rate.

The nitride semiconductor substrate described in the presentspecification is a substrate that contains at least Al_(x)Ga_(y)In_(z)N(where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1). The nitride semiconductorsubstrate may have about 20% or less of the nitrogen that it contains asan ingredient thereof replaced with at least one element selected fromthe group consisting of As, P, and Sb. Moreover, 10% or less (providedthat it has a hexagonal crystal) of the nitrogen contained in thesubstrate may be replaced with one element selected from the groupconsisting of As, P, and Sb. In the present specification, any suchsubstrate is called a GaN substrate.

The most preferable material for the nitride semiconductor substrate isGaN, which has a binary crystal. Using a binary crystal helps to obtaina substrate with a constant composition and stable characteristics, andalso helps to eliminate variation in composition during epitaxialgrowth. Using GaN helps to obtain good conductivity. Next in rank as apreferable material for the substrate comes AlGaN. Using a material,such as AlGaN, that has a lower refractive index than GaN makes possiblesatisfactory confinement of laser light in the active layer in caseswhere a semiconductor laser that emits light in an ultraviolet to blueregion of the spectrum is built with a material as described above.

The nitride semiconductor substrate may have impurities added thereto,such as an n-type or p-type dopant. Examples of such impurities include,among others, Cl, O, S, Se, Te, C, Si, Ge, Zn, Cd, Mg, and Be. Thepreferable total amount of impurities added is from 5×10¹⁶ cm⁻³to 5×10²⁰cm⁻³, both ends inclusive. Particularly preferable as an impurity addedto the nitride semiconductor substrate to give it n-type conductivity isone of Si, Ge, O, Se, and Cl.

The nitride semiconductor layer laid on top of the nitride semiconductorsubstrate described in the present invention is a layer that contains atleast Al_(x)Ga_(y)In_(z)N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1). Thenitride semiconductor layer thus laid may have about 20% or less of thenitrogen that it contains as an ingredient thereof replaced with atleast one element selected from the group consisting of As, P, and Sb.

The nitride semiconductor layer may have impurities added thereto, suchas an n-type or p-type dopant. Examples of such impurities include,among others, Cl, O, S, Se, Te, C, Si, Ge, Zn, Cd, Mg, and Be. Thepreferable total amount of impurities added is from 5×10¹⁶ cm⁻³ to5×10²⁰ cm⁻³, both ends inclusive. Particularly preferable as an impurityadded to the nitride semiconductor layer to give it n-type conductivityis one of Si, Ge, S, Se, and Cl. Particularly preferable as an impurityadded to the nitride semiconductor layer to give it p-type conductivityis one of Mg, Cd, and Be.

The active layer described in the present specification should beunderstood to denote any layer composed of a well layer alone or of oneor more well layers combined with barrier layers. For example, an activelayer having a single quantum well structure is composed of a singlewell layer alone or of a barrier layer, a well layer, and a barrierlayer; an active layer having a multiple quantum well structure iscomposed of a plurality of well layers and a plurality of barrierlayers.

In the present specification, a negative index in a formula indicating aplane or orientation of a crystal will be represented by a negative sign“−” followed by the absolute value of the index, instead of the absolutevalue accompanied by an overscore placed thereabove as required byconvention in crystallography, simply because the latter notation cannotbe adopted in the present specification. Likewise, such a planeorientation as has a polarity reverse to a particular plane orientation,such as the C, A, or M plane, will be represented by a negative sign “−”followed by the symbol of that particular plane orientation.

To achieve the above objects, according to the present invention, aplurality of nitride semiconductor layers are laid on top of a nitridesemiconductor substrate. Here, the nitride semiconductor layers include:an active layer having a quantum well structure by being composed of oneor more well layers and one or more barrier layers, and an acceptordoping layer. Moreover, the nitride semiconductor substrate includes, asa part thereof, a dislocation-concentrated region and, as all theremaining part thereof, a low-dislocation region. Furthermore, thenitride semiconductor layers laid immediately above thedislocation-concentrated region and the low-dislocation region have adepression immediately above the dislocation-concentrated region.

This depression separates adjacent portions of the low-dislocationregions, and thereby serves to effectively reduce the stresses andstrains that are produced between layers of different mixed crystalcompositions (for example, between an AlGaN layer used as a claddinglayer and other layers) included in the device structure. Thus, thestresses that are produced between a plurality of nitride semiconductorlayers having different lattice constants which are laid immediatelyabove the low-dislocation region are effectively reduced. This helps toreduce cracks, to make higher-quality epitaxial growth possible, and toenhance the light-emission characteristics, electrical characteristics,and life characteristics of the device.

For this device with enhanced characteristics to be produced with a highyield rate, it is preferable that, on the top surface of the nitridesemiconductor substrate, the area of the dislocation-concentrated regionbe smaller than the area of the low-dislocation region.

For easy formation of the depression immediately above thedislocation-concentrated region, the dislocation-concentrated region maybe the c-plane, which has a reverse polarity.

Moreover, slightly slanting the topmost surface of the nitridesemiconductor substrate, on top of which the device structure is laid,at an angle whose absolute value is in the range from 0.30° to 0.70°relative to the C plane offers the following benefits in addition tothose already mentioned above. Defects remaining in the substrate areprevented from spreading to the nitride semiconductor layers, and thesurface flatness of the device is improved, making higher-qualityepitaxial growth possible. This helps to reduce dislocations penetratingthe nitride semiconductor layers, to reduce current paths that do notcontribute to light emission, and to enhance the surface flatness of theprimer layer for the active layer. Thus, it is possible to realize anitride semiconductor laser device that offers a long life.

Moreover, the great reduction of defects in the nitride semiconductorlayers laid on top eliminates the need for an activation process such asheat treatment, because the activator doping layer as is grown readilyexhibits p-type conductivity.

When the plurality of nitride semiconductor layers constituting thedevice structure are laid on top of the substrate having thedislocation-concentrated and low-dislocation regions, by making thetotal thickness of the layers laid immediately above thedislocation-concentrated region equal to or smaller than half theirtotal thickness immediately above the low-dislocation region, it ispossible to separate, with the depression, adjacent portions of thelow-dislocation region including a high luminescence region so as tomake them discontinuous. This helps to greatly reduce the stressesapplied to and the resulting strains produced in the nitridesemiconductor layers immediately above the adjacent portions of thelow-dislocation region.

In this way, it is possible to free the active layer in the devicestructure from strains, and thereby to eliminate factors that degradethe device characteristics, such as phase separation or localagglomeration of In. Moreover, by separating adjacent portions of thelow-dislocation region, it is possible to effectively reduce cracks thatdevelop in the device when there are large strains, and thereby toincrease the production yield. Densely concentrated dislocations spreadfrom the substrate to the nitride semiconductor layers immediately abovethe dislocation-concentrated region, making it undesirable to form thedevice structure there. However, by reducing the total thickness of thenitride semiconductor layers immediately above thedislocation-concentrated region in advance in the stage in which theyare grown, it is possible to automatically make it impossible to formthe device structure there. This makes the screening of chips easy whenindividual chips are separated, and thus helps enhance the productionyield.

As described above, by forming the depression in the nitridesemiconductor layers immediately above the dislocation-concentratedregion and thereby separating adjacent regions immediately above thelow-dislocation region, it is possible to improve the devicecharacteristics and the yield simultaneously. However, if the planeorientation of the substrate surface is the C plane, surfaceirregularities in the shape of hexagonal cones may develop on thesurface of the grown nitride semiconductor layers. Development of thesesurface irregularities may degrade the steepness and planar uniformityof the active layer.

To avoid this, the substrate surface is slanted at an angle whoseabsolute value is in the range from 0.30° to 0.70° relative to the Cplane. This helps to enhance flatness. The mechanism working here isbelieved to be as follows. At an early stage of the growth of thenitride semiconductor layers, a growth nucleus is formed starting at akink of the terrace structure present on the substrate surface slantedat an angle whose absolute value is in the range from 0.30° to 0.70°relative to the C plane. Then, lateral growth is performed, starting atthe growth nucleus, uniformly in the direction of the terrace. Thishelps to realize satisfactory two-dimensional growth, and eventually tomake flat the surface after film formation.

Meanwhile, irrespective of whether growth proceeds two-dimensionally ornot, dislocations present in the substrate spread into the layers beinggrown. Thus, to eliminate the effects of the dislocation-concentratedregion and thereby to increase the yield in the cleaving process, it iseffective, as described above, to reduce the total thickness of thelayers grown immediately above the dislocation-concentrated region.

Moreover, by slanting the substrate surface at an angle whose absolutevalue is in the range from 0.30° to 0.70° relative to the C plane, it ispossible to obtain an optimum, uniform terrace. This helps to achievetwo-dimensional growth in which the materials in a gas phase reach thesubstrate surface and, while repeating migration and re-evaporation,uniformly form the growth nucleus, depositing one layer on top ofanother on the surface. This helps to produce a device that has reducedthreading dislocations, that achieves efficient current injection, thatproduces reduced heat, and that has a surface with enhanced flatness.Moreover, it is possible to realize enhanced crystallinity and a uniformcomposition in the light-emitting layer containing In. In this way, itis possible to enhance the light-emitting characteristics of the deviceand to extend the life thereof.

As a result of the reduction of defects in the acceptor doping layerincluded in the nitride semiconductor multiple-layer film laid on top ofthe substrate, the acceptor impurities are no longer captured by suchdefects, and are therefore less likely to be deactivated by hydrogen.Thus, even without an activation process such as heat treatment, theacceptor doping layer as is grown readily exhibits p-type conductivity,with a hole density of 10¹⁷ cm⁻³ or more. This makes it possible toreduce the heat history applied to the active layer, and thus to producethe device without degrading the active layer and without degrading thedevice characteristics. Here, there is no particular restriction on thedirection in which the substrate surface is slanted relative to the Cplane (the<0001> direction), though the<11-20> or <1-100> direction ispreferred.

Slanting the substrate surface helps to suppress the spreading ofdefects into the device structure laid on top thereof, and thereby helpsimprove the flatness of the surfaces of the individual layers,permitting steep interfaces to be formed therebetween. This effectcontributes to enhanced quality of, in particular, the active layer.Usually, in an active layer having a quantum well structure, theindividual well layers and barrier layers are as thin as about severalnm, and therefore the uniformity of the film thickness of the individuallayers is greatly affected by the surface condition of the primer layer.If the surface flatness of the layer immediately under the active layeris, for example, several nm, which is about equal to the thickness of awell layer or barrier layer, place-to-place variations of the devicecomposition, thickness, or the like of the quantum well structuredisturb uniformity, resulting in nonuniform quantum level, which affectsthe light-emission characteristics. According to the present invention,the substrate surface is slanted at an angle whose absolute value is inthe range from 0.3° to 0.7° relative to the C plane. This permits theindividual layers constituting the device to be formed flat, and therebymakes it possible to solve the problems mentioned above.

To satisfactorily enhance the flatness of the well layers and barrierlayers that constitute the active layer, it is advisable to make thetotal thickness of all the layers laid between the substrate and theactive layer equal to or greater than 1 μm. This helps to make theabove-described effect more noticeable. At least one of the elementselected from the group consisting of As, P, and Sb may be added to theactive layer.

Advisably, the substrate is made of a nitride semiconductor, and has, asa part thereof, a dislocation-concentrated region and, as all theremaining part thereof, a low-dislocation region. This helps to greatlyreduce the density of dislocations present in the low-dislocation regionthan in a common substrate, and thus helps to make the above-describedeffect still more noticeable.

In a semiconductor optical device according to the present invention, anitride semiconductor laser device as described above is used as a lightsource.

To achieve the above objects, according to the present invention, in anitride semiconductor light-emitting device having a plurality ofnitride semiconductor layers laid on top of a nitride semiconductorsubstrate, the nitride semiconductor substrate includes, as a partthereof, a stripe-shaped defect-concentrated region in which crystaldefects concentrate and, as all the remaining part thereof, a low-defectregion, and the top surface of the nitride semiconductor substrate hasan off-angle in the direction perpendicular to the direction of thestripe of the defect-concentrated region.

As described above, in the present specification, in a nitridesemiconductor substrate, the stripe-shaped region where defectsconcentrate as a result of the positions of defects controlled asdesired is called the dislocation-concentrated region, and the regionwhere the concentration of defects is low which is obtained as a resultof concentrating the positions of defects in thedislocation-concentrated region is called the low-dislocation region.

By forming the nitride semiconductor layers on the top surface of thenitride semiconductor substrate which has an off-angle in the directionperpendicular to the stripe of the defect-concentrated region, it ispossible to obtain, on the top surface of the nitride semiconductorsubstrate, a depression-like region where light emission is less uniformabove one of the low-defect regions located on both sides of thedislocation-concentrated region. In the present specification, thisregion where light emission is less uniform is called theuniformly-light-emitting region. Incidentally, the direction in which alow-defect region is obtained with respect to a defect-concentratedregion depends on the off-angle in the direction perpendicular to thestripe of the defect-concentrated region.

According to the present invention, in the nitride semiconductorlight-emitting device described above, the off-angle is in the rangefrom 0.2° to 2.0°, both ends inclusive.

When the top surface of a nitride semiconductor substrate having adefect-concentrated region has an off-angle in the range from 0.2° to2.0°, both ends inclusive, with respect to the stripe-shapeddefect-concentrated region, a uniformly-light-emitting region isobtained in the nitride semiconductor layers formed on top of thenitride semiconductor substrate. This uniformly-light-emitting regiondoes not always have a constant width; the width and depth of theuniformly-light-emitting region vary with the off-angle. Specifically,the tendency is that, the gentler the off-angle, the greater the widthand the smaller the depth; by contrast, the sharper the off-angle, thesmaller the width and the greater the depth. Moreover, theuniformly-light-emitting region is formed within a range of 50 μm ormore but 200 μm or less of the edge of the defect-concentrated region.However, an off-angle of 0.2° or less is undesirable because it makesthe uniformly-light-emitting region unlikely to appear, and an off-angleof 2.0° or more is also undesirable because it gives theuniformly-light-emitting region too small a width.

According to the present invention, in the nitride semiconductorlight-emitting device described above, the top surface of the nitridesemiconductor substrate has an off-angle in the direction parallel tothe direction of the stripe of the defect-concentrated region, and theoff-angle is 2° or less.

The off-angle in the direction parallel to the stripe-shapeddefect-concentrated region may be just zero (0°), but is preferably 2°or less. The reason is that, when the top surface of the GaN substratehas an off-angle in the direction parallel to the stripe of thedefect-concentrated region, it is possible to make substantiallyconstant the width of the uniformly-light-emitting region in the surfaceof the nitride semiconductor layers formed on top of the nitridesemiconductor substrate. However, it is desirable that the off-angle inthe direction parallel to the stripe of the defect-concentrated regionbe smaller than the off-angle in the direction perpendicular thereto.The reason is that there is a tendency that, when the off-angle in theperpendicular direction is sharp, the uniformly-light-emitting region isnarrow, and that, when the off-angle in the parallel direction isgreater than that in the perpendicular direction, theuniformly-light-emitting region is obtained in a narrower region.

According to the present invention, a ridge-stripe portion or anarrowed-current portion is formed on the uniformly-light-emittingregion. This helps reduce variations in the device characteristics, suchas the wavelength, of individual chips separated, and thus helpsincrease the yield. However, in a region within about 30 μm of thedefect-concentrated region, the top surface of the nitride semiconductorlayers is elevated and its width is not constant. This makes itdifficult to form a ridge-stripe portion or a narrowed-current portionthere. Accordingly, this region needs to be avoided.

As described above, by growing a nitride semiconductor laser devicestructure on top of a GaN substrate having a stripe-shapeddefect-concentrated region and having a predetermined off-angle, it ispossible to obtain a uniformly-light-emitting region. By forming aridge-stripe portion or a narrowed-current portion on this region, it ispossible to realize, with a high yield, a nitride semiconductorlight-emitting device with less defects, with an improved useful life,and with high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 a sectional view schematically showing the semiconductor laserdevice of Embodiment 1 of the invention;

FIG. 2(a) is a sectional view of the nitride semiconductor substrate inthe fabrication procedure, FIG. 2(b) a perspective view schematicallyshowing the ingot, FIG. 2(c) a sectional view of the nitridesemiconductor substrate, FIG. 2(d) a top view of the nitridesemiconductor substrate, and FIG. 2(e) a top view of the nitridesemiconductor substrate;

FIG. 3 is a sectional view schematically showing the layer structure ofthe semiconductor laser device of Embodiment 1;

FIG. 4(a) is a sectional view schematically showing how a nitridesemiconductor layer grows on a substrate with no surface inclination;FIG. 4(b) is a sectional view schematically showing a grown nitridesemiconductor layer on a substrate with a surface inclination, at anearly stage of the growth; and FIG. 4(c) is a sectional viewschematically showing a grown nitride semiconductor layer on a substratewith a surface inclination, on completion of the growth;

FIG. 5 is a diagram showing the relationship between the inclination ofthe substrate surface relative to the C plane and the mean surfaceroughness in the nitride semiconductor laser device of Embodiment 2 ofthe invention;

FIG. 6 is a diagram showing the relationship between the inclination ofthe substrate surface relative to the C plane and the FWHM of emittedlight in the LED mode in the nitride semiconductor laser device ofEmbodiment 2;

FIG. 7 is a diagram showing the relationship between the inclination ofthe substrate surface relative to the C plane and the EPD within a 200μm width of the dislocation-concentrated region in the nitridesemiconductor laser device of Embodiment 2;

FIG. 8 is a sectional view schematically showing the substrate and thegrown nitride semiconductor layer in Embodiment 3 of the invention;

FIG. 9 is a diagram showing the relationship between the grown nitridesemiconductor layer thickness and the mean surface roughness inEmbodiment 3;

FIG. 10 is a sectional view schematically showing the layer structure ofthe semiconductor laser device of Embodiment 5 of the invention;

FIG. 11 is a block diagram showing an outline of the construction of thesemiconductor optical apparatus of Embodiment 8 of the invention;

FIG. 12 is a top view of the wafer having the ridge-stripe portionsformed thereon in Embodiment 9 of the invention;

FIG. 13 is a sectional view of the GaN substrate in Embodiment 9;

FIG. 14 is a sectional view of the laser diode device in Embodiment 9;

FIG. 15(a) is a sectional view of a principal portion of the supportbase member having the n-type GaN layer laid thereon; FIG. 15(b) is adiagram showing the ingot; FIG. 15(c) is a sectional view of a principalportion of the n-type GaN layer; and FIG. 15(d) is a top view of aprincipal portion of the n-type GaN layer;

FIG. 16 is a sectional view of a principal portion of a GaN substratehaving an off-angle relative to the C plane;

FIG. 17 is a sectional view of the GaN substrate in Embodiment 10 of theinvention;

FIG. 18 is a top view of the wafer having the ridge-stripe portionsformed thereon in Embodiment 10;

FIG. 19 is a sectional view of the laser diode device in Embodiment 10;

FIG. 20 is a sectional view of the GaN substrate in Embodiment 11 of theinvention; and

FIG. 21 is a top view of the wafer having the ridge-stripe portionsformed thereon in Embodiment 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

Embodiment 1

FIG. 1 is a sectional view schematically showing the semiconductor laserdevice 1 of Embodiment 1 of the invention. This figure shows thesemiconductor laser device 1 of this embodiment as seen from thedirection in which it emits light. In FIG. 1, reference numeral 101represents an n-type GaN substrate of which the topmost surface isinclined slightly, specifically in the range from 0.30° to 0.70° inabsolute-value terms, relative to the C-plane. The substrate 101includes, as a part thereof, a dislocation-concentrated region 102, allthe remaining part of the substrate 101 being a low-dislocation region.The low-dislocation region in turn includes, as a part thereof, ahigh-luminescence region 103, which runs parallel to thedislocation-concentrated region 102. On top of the substrate 101, anitride semiconductor layer (epitaxially grown layer) 104 is formed. Thenitride semiconductor layer 104 has a laser light waveguide region 105formed inside it, and has a depression 108 formed immediately above thedislocation-concentrated region 102. On the top surface of the nitridesemiconductor layer 104 and on the bottom surface of the substrate 101,there are formed electrodes 106 and 107, respectively.

Now, a description will be given of the fabrication procedure of thesemiconductor laser device of this embodiment, with more detailedexplanations of the structure thereof given whenever appropriate.

Fabrication Procedure of the GaN Substrate

First, an outline of how the crystal of the n-type GaN substrate 101 isgrown will be described. A GaN crystal is grown with facets kept exposedas inclined surfaces so that the inclined surfaces are maintained. Thatis, as crystal growth proceeds, the inclined surfaces are graduallymoved in the direction of the growth. This permits dislocations thatappear in the middle portions of the inclined surfaces to spread to andaggregate at the lower ends of the inclined surfaces, formingdislocation-concentrated regions where the lower ends of the inclinedsurfaces were located and low-dislocation regions where the middleportions of the inclined surfaces were located.

The dislocation-concentrated regions can be in one of different states:they may be polycrystalline; or they may be monocrystalline, slightlyinclined relative to the surrounding low-dislocation regions; or theymay have the opposite growth direction [000-1] to the growth direction[0001] of the surrounding low-dislocation regions. This produces clearboundaries between the dislocation-concentrated regions and thelow-dislocation regions.

Since the inclined surfaces are moved in the growth direction, byappropriately producing the facets that appear at the beginning, it ispossible to determine the shape of the dislocation-concentrated regions.If facets like the side faces of an inverted pyramid (with its vertexdown and base up) are produced at the beginning, dislocationsconcentrate at the vertices of such pyramids, and thus thedislocation-concentrated regions are formed in straight lines parallelto the growth direction, forming pits. If facets like the side faces ofa groove with a V-shaped section are produced at the beginning,dislocations concentrate at the bottoms of such grooves (the linearparts thereof), and thus the dislocation-concentrated regions are formedin a plane parallel to the growth direction, forming stripes.

As a seed of the facets that are produced at the beginning, a mask thatimpedes crystal growth can be used. Crystal growth starts where no maskis present, and thus facets are produced at the boundaries between whereno mask is present and where the mask is present. As crystal growthproceeds in the horizontal direction, the facets meet on the mask, andat those meeting points the dislocation-concentrated regions start toform.

After the facets meet, crystal growth proceeds stably in the verticaldirection (the very direction in which crystal growth is inherentlysupposed to proceed), and thus the facets move, as they are, in thegrowth direction, permitting the dislocation-concentrated regions toextend in the growth direction. If the mask that impedes crystal growthis given a pattern of dots, facets like the side faces of an invertedpyramid are produced; if the mask is given a pattern of straight lines,facets like the side faces of a groove with a V-shaped section areproduced. The mask may be an amorphous or polycrystalline layer, and isformed, for example, by laying a thin film of SiO₂ on the surface of abase member.

After crystal growth, the surface is specularly polished so that thetopmost surface is inclined slightly, specifically in the range of 0.30°to 0.70° in absolute-value terms, relative to the C plane. In this way,it is possible to obtain a substrate on which a high-quality nitridesemiconductor layer can be laid.

This embodiment deals with a substrate that has dislocation-concentratedregions arranged in a pattern of stripes or of dots. That is, eitherfacets are produced in a V shape so that dislocation-concentratedregions are formed in a pattern of stripes, or a large number of facetsso shaped as to surround dots are produced at short intervals so thatdislocation-concentrated regions are formed, when observedmacroscopically, in a pattern of stripes. Alternatively, a large numberof facets so shaped as to surround dots are produced at long intervalsin a nested fashion so that dislocation-concentrated regions are formedin a pattern of discrete dots.

Next, a practical example of the fabrication procedure of the n-typesubstrate 101 will be described with reference to FIG. 2. On a supportbase member 201, an n-type GaN layer 202 is grown by HVPE in such a waythat chiefly the {11-22} facet 203 is exposed at the surface duringgrowth. This causes the surface to have a sawtooth-shaped section withridges and troughs. Near the tops of the ridges, however, was slightlyexposed the {0001} plane 206 in the shape of stripes. This isillustrated in the (partial) sectional view in FIG. 2(a).

Here, HVPE is a process that involves the following operations. In aboat provided in an upstream part of a hot-wall reactor furnace, HCL gasis blown into heated molten Ga to produce GaCl, which is then sentdownward. In a downstream part of the reactor furnace, NH₃ is blown intoa support base member placed there. Thus, in the downstream part, thetwo react, depositing GaN on the base member.

Used as the support base member 201 was a two-inch (111) GaAs wafer.There is no particular restriction on the material of the base member201, and it is possible to use sapphire as commonly used. However, foreasy separation of the base member from the GaN deposited thereon, it ispreferable to use GaAs or the like to minimize waste.

The ridges and troughs mentioned above have a periodic structure with apitch P of 400 μm, and have a shape that extends, like ridges in a cropfield, in the depth direction of the figure. To determine where tolocate these ridges and troughs, a mask of SiO₂ or the like may beformed beforehand on the base member 201 where the troughs are supposedto run so that crystal growth proceeds while the mask is used as a seedof the facets that are exposed. That is, the mask has a pattern ofstripes arranged parallel to the [1-100] direction of the GaN crystalwith a pitch P of 400 μm. Thus, the mask has the shape of continuousstripes, or of a large number of dots located at substantially regularintervals on straight lines.

The technique of (and the conditions for) maintaining crystal growthwith the {11-22} facet kept exposed is disclosed in detail in JapanesePatent Application Laid-Open No. H11-273882, which the applicant of thepresent invention once filed. The grown crystal was made n-type by beingdoped with O while growing.

By continuing the growth of the GaN crystal with this growth modemaintained, a 30 mm high ingot was produced on top of the base member201. FIG. 2(b) schematically shows the ingot.

Using a slicer, this ingot was sliced into thin pieces (each an n-typeGaN substrate). The thin piece was then polished to produce aflat-surfaced n-type GaN substrate 101 measuring two inches (about 5 cm)in diameter and 350 μm in thickness. The surface for epitaxial growthwas specularly polished. At the surface appeared largely the (0001)plane, but here it was preferable that the surface be inclined at acomparatively small angle, specifically in the range of 0.2° to 1° inabsolute-value terms, in any direction relative to the (0001) plane.This helps effectively prevent the spreading of defects from thesubstrate to the nitride semiconductor layer 104 epitaxially grownthereon and thereby achieve epitaxial growth with satisfactory surfacemorphology. In particular, for maximum surface flatness, it waspreferable that the inclination angle be in the range of 0.30° to 0.70°in absolute-value terms.

FIG. 2(c) is a sectional view (showing a part) of the n-type substrate101 obtained in this way, and FIGS. 2(d) and 2(e) are top views (showinga part) thereof. FIG. 2(d) schematically shows the surface of asubstrate on which dislocation-concentrated regions 204 andhigh-luminescence regions 205 are arranged in a pattern of stripes. FIG.2(e) schematically shows the surface of a substrate on whichdislocation-concentrated regions 204 are formed in a pattern of dots orcircles, low-dislocation regions are formed in a pattern of dodecagons,and high-luminescence regions 205 are arranged in the gaps across whichthe low-dislocation regions adjoin. Within the substrate surface, thedislocation-concentrated regions 204, low-dislocation regions, andhigh-luminescence regions 205 may be arranged in any other manner thanin the examples shown in FIGS. 2(d) and 2(e). For example, thedislocation-concentrated regions may be arranged in a pattern of brokenlines. By appropriately selecting the mask pattern used at an earlystage of growth, it is possible to adjust the individual regions on thesubstrate.

The GaN substrate 101 thus obtained was evaluated in the followingmanner. First, the surface of the GaN substrate 101 was closelyinspected under a microscope. Even though polished, the surface was notperfectly flat, but had small depressions in regions corresponding tothose where the bottommost parts of the troughs were located duringcrystal growth (the regions indicated by reference numeral 204 in FIG.2(a)).

Then, the sample was etched by being dipped in a mixture of sulfuricacid and phosphoric acid heated to 250° C. so that etch pits appeared atthe surface. As a result, in regions corresponding to those where thebottommost parts of the troughs were located during crystal growth (theregions indicated by reference mark 204 in FIG. 2(a)), many etch pitsappeared, attesting that those regions were where dislocations (orcrystal defects) were highly concentrated (i.e.,dislocation-concentrated regions). That is, the depressions mentionedabove corresponded to these regions.

As described above, the regions in which depressions were observed hadhighly concentrated dislocations, and thus were more easily eroded thanother regions. This probably is the reason that the depressions wereformed there. The dislocation-concentrated regions were about 10 to 40μm wide. All the other regions were low-dislocation regions with EPDs(etch pit densities) of the order of 10⁴ to 10⁵ cm⁻². Thedislocation-concentrated regions were observed to have three or moreorders of magnitude greater EPDs. Thus, the regions indicated byreference numeral 102 are where the dislocation density is severalorders of magnitude higher than elsewhere, and this is the reason thatthose regions are called “dislocation-concentrated regions” in thepresent specification.

Moreover, with the sample irradiated with ultraviolet rays (the 365 nmspectral line of a Hg lamp can be used), the luminescence from thesurface was observed under a microscope (fluorescence microscopy). As aresult, in the middle portions of the low-dislocation regions sandwichedbetween the dislocation-concentrated regions 102, stripe-shaped regionswere observed that had comparatively clear boundaries and that haddifferent contrast from the adjoining regions. These regions exhibitmore intense luminescence, which can be observed with the naked eye,than the adjoining regions, and are observed to be yellowish and bright.

These regions are the parts 203 which grew with the {0001} facet exposedduring crystal growth, and are thus the regions indicated by referencenumerals 103 and 205 in FIGS. 1 and 2(d), respectively. One of theprobable reasons that these regions are observed to appear differentfrom the adjoining regions is that the dopant is absorbed differentlythere than in the adjoining regions. On the basis of the facts describedabove, in the present specification, these regions are called“high-luminescence regions.” During crystal growth, the parts 203 thatgrow with the {0001} facet exposed do not always advance uniformly witha constant width. Thus, the high-luminescence regions 205 had slightlyfluctuating widths, about 30 μm at the maximum.

Hardly any high-luminescence regions may be formed depending on theconditions under which the above-mentioned ingot is produced and on theposition within the ingot (i.e., the distance from the support basemember). However, the substantially middle regions of the partssandwiched between the dislocation-concentrated regions 204 correspondto the regions near the tops of the above-mentioned ridges and troughs,and, in the present specification, these regions are also called“low-dislocation region middle regions.” In the present specification,any description mentioning the high-luminescence regions should beunderstood to mention the low-dislocation region middle regions.

The formation of the GaN substrate 101 may be achieved by anyvapor-phase crystal growth process other than HVPE; it may be formed,for example, by MOCVD (matalorganic chemical vapor deposition), MOC(matalorganic chloride vapor phase epitaxy), or sublimation.

Used as the base member 301 on which to grow the GaN substrate 101 maybe, instead of GaAs, a crystal substrate that has sixfold or threefoldsymmetry about an axis. That is, a hexagonal or cubic crystal can beused. With a cubic crystal, using the (111) plane results in threefoldsymmetry. Examples of hexagonal crystals that can be used include SiC,SiO₂, NdGaO₃, ZnO, GaN, and AlN. Sapphire, of which the structure is,strictly speaking, rhombohedral but very close to hexagonal, can also beused. Examples of cubic crystals of which a (111) plane substrate can beused include Si, spinel, MgO, and GaP. With these, GaN is grown on the Cplane.

The mask that is used in the formation of the GaN substrate 101 is laidby one of the following two methods. By one method, the mask is formeddirectly on the base member. In this case, it is advisable to take apreparatory step prior to epitaxial growth as by depositing a GaN bufferlayer on the surface where the base member will be exposed through theopenings in the mask. By the other method, a comparatively thin GaNlayer is formed beforehand on the base member, and then the mask isformed on top. In many cases, the latter method proves to be preferable,permitting smooth progress of growth.

The embodiment being described deals with a case where GaN is used asthe substrate 101, but the material thereof may be replaced with anymentioned earlier in the “SUMMARY OF THE INVENTION” section.

In the procedure for fabricating a nitride semiconductor substratedescribed earlier in the “Description of the Prior Art” section, as thegrowth of the substrate crystal proceeds, a lateral growth technique(ELOG) is used to reduce dislocations. However, there, the positions ofdislocations (defects) are not controlled, with the result that, ascrystal growth proceeds, dislocations come to be distributed uniformlyin a plane. By contrast, in the nitride semiconductor substrate used inthe present invention, throughout the growth of the substrate crystal,the positions of dislocation-concentrated regions are controlled so thatthey are located at predetermined positions (with a pitch of the orderof several hundred μm). This is the difference between the prior-art GaNsubstrate and the nitride semiconductor substrate used in the presentinvention.

Accordingly, to produce a substrate with a given dislocation density,the procedure for fabricating the substrate crystal described inconnection with the currently described embodiment requires less crystalgrowth steps, leading to higher productivity.

Epitaxial Growth of the Nitride Semiconductor Layer

Next, how the nitride semiconductor layer 104 etc. are formed on top ofthe n-type GaN substrate 101 to fabricate the semiconductor laser device1 will be described with reference to FIGS. 3 and 4. FIG. 3 is a diagramschematically showing the semiconductor laser device 1 of FIG. 1, withthe layer structure of the nitride semiconductor layer 104 illustratedin detail. In this figure, the already-described structure within thesubstrate 101 is omitted. FIG. 4 is a diagram schematically showing howgrowth proceeds when a nitride semiconductor layer is grown on theC-plane substrate of GaN and on a GaN substrate of which the surface isinclined at an angle in the range of 0.30° to 0.70° in absolute-valueterms relative to the C plane.

Using a MOCVD machine, SiH₄ as a dopant material was added to NH₃ as agroup V material and TMGa (trimethyl gallium) or TEGa (triethyl gallium)as a group III material, and, at a substrate temperature of 1,050° C., a3 μm thick n-type GaN layer 301 was formed on the n-type GaN substrate101. As described in the “Fabrication Procedure of the GaN Substrate”section above, the substrate 101 may have dislocation-concentratedregions arranged in a pattern of stripes, or may have them arranged in apattern of dots in a nested fashion. In a case where thedislocation-concentrated regions has a pattern of dots, they do notnecessarily have to be arranged in a nested fashion. More specifically,so long as the substrate 101 has dislocation-concentrated regions in astate clearly distinguishable from low-dislocation regions and inaddition, when seen from the direction perpendicular to the surface onwhich to grow a nitride semiconductor layer, thedislocation-concentrated regions have a smaller area than thelow-dislocation regions, it can be used satisfactorily irrespective ofthe arrangement of the individual dislocation-concentrated regions.

After the formation of the n-type GaN layer 301, TMIn (trimethyl indium)as a group III material was added to the above-mentioned materials, and,at a substrate temperature of 800° C., a 40 nm thick n-typeIn_(0.07)Ga_(0.93)N crack prevention layer 302 was grown. Next, thesubstrate temperature was raised to 1,050° C., and, using also TMAl(trimethyl aluminum) or TEAl (triethyl aluminum) as a group IIImaterial, a 1.2 μm thick n-type Al_(0.1)Ga_(0.9)N cladding layer 303 wasgrown. As an n-type impurity, 5×10¹⁷ to 1×10¹⁹ of Si was added.Subsequently, a 0.1 μm thick n-type GaN light guiding layer 304 (with aSi impurity concentration of 10¹⁶ to 10¹⁸ cm⁻³) was grown. It has beenknown that the total thickness of the layers thus far grown needs to be1 μm or more to give the layer (in this embodiment, the n-type GaN lightguiding layer) immediately below the active layer sufficient flatness toenhance the uniformity and quality of the active layer.

Thereafter, the substrate temperature is lowered to 750° C., and anactive layer (multiple quantum well structure) 305 composed of 4 nmthick In_(0.1)Ga_(0.9)N well layers and 8 nm thick In_(0.01)Ga_(0.99)Nbarrier layers that are laid alternately on one another in three periodsin the following order: a barrier layer, a well layer, a barrier layer,a well layer, a barrier layer, a well layer, and then a barrier layer.Meanwhile, SiH₄ (with a Si impurity concentration of 10¹⁶ to 10¹⁸ cm⁻³)was added to the barrier layers or to both the barrier and well layers.It is preferable to suspend the growth for a period of one second ormore but less than 180 seconds between adjacent barrier and well layersor between adjacent well and barrier layers, because this helps enhancethe flatness of the individual layers and helps reduce the FWHM (fullwidth at half maximum) of emitted light.

A preferred material used to add As to the active layer is AsH₃ (arsine)or TBAs (tertiary butyl arisine); a preferred material used to add P tothe active layer is PH₃ (phosphine) or TBP (tertiary butyl phosphine); apreferred material used to add Sb to the active layer is TMSb (trimethylantimony) or TESb (triethyl antimony). Used as the N material in theformation of the active layer may be, instead of NH₃, N₂H₄ (hydrazine),C₂N₂H₈ (dimethyl hydrazine), or an organic material containing N.

Next, the substrate temperature was raised back to 1,050° C., and a 20nm thick p-type Al_(0.3)Ga_(0.7)N carrier blocking layer 306, a 0.1 μmthick p-type GaN light guiding layer 307, a 0.5 μm p-typeAl_(0.1)Ga_(0.9)N cladding layer 308, and a 0.1 μm thick p-type GaNcontacting layer 309 were grown in this order. As a p-type impurity,EtCp₂Mg (bisethylcyclopentadienyl magnesium) was used as a material, andwas added so that the Mg concentration was 10¹⁸ to 2×10²⁰ cm⁻³. It ispreferable that the p-type impurity concentration in a p-type GaNcontacting layer 310 increase toward a p-electrode 311. This helpsreduce the contact resistance due to the formation of the p-electrode.Moreover, to eliminate the residual hydrogen in the p-type layers whichimpedes the activation of Mg used as the p-type impurity, a trace amountof oxygen may be mixed during the growth of the p-type layers.

After the p-type GaN contacting layer 309 was grown in this way, all thecontents inside the reactor of the MOCVD machine were replaced withnitrogen carrier gas and NH₃, and the temperature was lowered at a rateof 60° C. per minute. When the substrate temperature became 800° C., thesupply of NH₃ was stopped, and then, after this substrate temperaturewas maintained for five minutes, it was lowered further to roomtemperature. Here, a preferred range of temperature at which thesubstrate is maintained is from 650° C. to 900° C., and a preferredrange of duration for which that temperature is maintained is from threeto ten minutes. A preferred rate at which the temperature is lowered is30° C. or more per minute.

Now, how a nitride semiconductor layer grows on a C-plane substrate andon a substrate of which the surface is inclined at an angle in the rangeof 0.30° to 0.70° in absolute-value terms relative to the C plane willbe described with reference to FIG. 4. For the sake of explanation, FIG.4 includes some exaggeration. As shown in FIG. 4(a), when a nitridesemiconductor layer 404 is grown on a C-plane substrate 401 that has nosurface inclination and that includes dislocation-concentrated regions402 and high-luminescence regions 403 inside it, elevations anddepressions may develop on the surface of the grown layer 404. Thisindicates that two-dimensional growth is insufficient elsewhere than inthe dislocation-concentrated regions 402. The elevations and depressionsare, for example, projections in the shape of hexagonal prisms, andproduce a rough surface.

Moreover, crystal growth is slower immediately above thedislocation-concentrated regions 402, because there the surface isterminated with nitrogen atoms. Thus, in these areas, the grown layerhas a smaller total thickness, causing crystal growth to proceed in sucha way as to produce depressions 405 right above thedislocation-concentrated regions 402. When two-dimensional growth isinsufficient, as the thickness of the grown layer 404 increases, thedislocation-concentrated regions 402 present inside the substrateenlarge and spread into the grown layer to produce obliquedislocation-concentrated regions 406. The precise mechanism working hereis unknown, but a probable explanation is that insufficienttwo-dimensional growth causes dislocations, which are inherentlysupposed to spread vertically, to spread in oblique directions,resulting in enlargement of dislocation-concentrated regions.

By contrast, as shown in FIG. 4(b), in a substrate 407 that includesdislocation-concentrated regions 402 and high-luminescence regions 403inside it and that has a surface inclination, its surface, when observedmicroscopically, has terrace-shaped kinks 408 arranged at regularintervals. When this substrate 407 is placed on a MOCVD machine and anitride semiconductor layer is grown thereon, at an early stage ofgrowth, kinks 404 that are present regularly on the surface of thesubstrate serve as the main starting points of crystal growth. Thus, forexample, the group III material such as Ga, by repeating migration andabsorption, starts to grow laterally from the kinks. This mode of growthis called two-dimensional growth.

When the lateral growth proceeds until it reaches the next kinks, theentire substrate surface is covered with a grown nitride semiconductorlayer 409, and now a new layer starts to grow two-dimensionally. This isrepeated so that, as time passes, growth proceeds in the thicknessdirection as shown in FIG. 4(c). In this way, two-dimensional growthproceeds regularly with kinks 408 due to a slight inclination of thesubstrate surface serving as starting points, and thus, when the grownnitride semiconductor layer 409 has grown to a certain thickness, itssurface has sufficient flatness to produce with high quality a multiplequantum well active layer that is composed of many well and barrierlayers, each several nm thick, on one another. Since, in thedislocation-concentrated regions 402, the surface is terminated withnitrogen atoms, growth is slower there, and accordingly, immediatelyabove the dislocation-concentrated regions 402, depressions 410 remaineven as growth proceeds.

Giving the substrate surface an inclination in the range of 0.30° to0.70° relative to the C plane permits the dislocation-concentratedregions 402 present inside the substrate to spread vertically to thegrown nitride semiconductor layer 409, and thus prevents dislocationsfrom spreading into the grown nitride semiconductor layer 409. As aresult of the foregoing, Giving the substrate surface an inclination inthe range of 0.30° to 0.70° relative to the C plane helps enhance theflatness of the surface of the grown nitride semiconductor layer 409,and permits epitaxial growth of the nitride semiconductor layer withoutenlarging the dislocation-concentrated regions 402 inside the substrate.

The grown film thus produced was evaluated by Raman measurement, withthe following results. After the wafer was taken out of the MOCVDmachine, even without p-typifying annealing, the grown film exhibitedp-type properties (the activation of Mg) immediately after growth. Inaddition, the contact resistance due to the formation of the p-electrodewas low. Combining the above technique with conventional p-typifyingannealing proved to be preferable, because it led to enhanced Mgactivation.

Moreover, because of the depressions 410 resulting from the slowergrowth immediately above the dislocation-concentrated regions 402 wherethe substrate surface was terminated with nitrogen atoms, the grownnitride semiconductor layer 409 was divided at the pitch betweenadjacent dislocation-concentrated regions. This helped reduce theresidual stress in the device, and thus helped achieve a lower crackdensity than in the conventional device structure, leading to anincreased yield. This permits the In_(0.07)Ga_(0.93)N crack preventionlayer 302 to have an In content other than 0.07, and permits omission ofthe entire InGaN crack prevention layer.

The active layer 305 starts with a barrier layer and ends with a barrierlayer; instead, it may start with a well layer and end with a welllayer. The number of well layers formed may be any other than three asspecifically described above; so long as the number is ten or less, alow threshold current density permitted continuous wave operation atroom temperature. Two or more but six or less layers were particularlypreferable, because of a low threshold current density. The active layerdescribed above may further contain Al.

Here, a necessary amount of Si was added to both the well and barrierlayers that constituted the active layer 305. This impurity does notnecessarily have to be added, but adding an impurity such as Si to theactive layer resulted in more intense light emission. The impurity maybe, other than Si, O, C, Ge, Zn, or Mg used singly, or two or more ofthem used in combination. A preferred total amount of impurities addedwas about 10¹⁷ to 8×10¹⁸ cm⁻³. The impurity may be added to, instead ofboth the well and barrier layers, the well layers alone or the barrierlayers alone.

The p-type Al_(0.3)Ga_(0.7)N carrier blocking layer 306 may have acomposition other than this specific composition. Using AlGaN having Inadded thereto is preferable, because it becomes p-type even when grownat a lower temperature, and thus helps alleviate the damage inflicted onthe active layer 305 during crystal growth. The entire carrier blockinglayer 306 may be omitted, but providing it resulted in a lower thresholdcurrent density. This is because the carrier blocking layer 306 servesto confine carriers inside the active layer 305. Increasing the Alcontent in the carrier blocking layer 306 is preferable, because itpermits carriers to be confined more powerfully. By contrast, reducingthe Al content down to a level that barely maintains confinement ofcarriers is also preferable, because it increases the carrier mobilityinside the carrier blocking layer and thereby reduces the electricalresistance.

Here, an Al_(0.1)Ga_(0.9)N crystal was used as each of the p-typecladding layer 308 and the n-type cladding layer 303. Instead, an AlGaNternary crystal with an Al content other than 0.1 may be used. As the Alcontent in the mixed crystal increases, the differences in energy gapand in refractive index from the active layer 305 increase. This permitsmore efficient confinement of carriers and light in the active layer,leading to a lower threshold current density. By contrast, reducing theAl content down to a level that barely maintains the confinement ofcarriers and light, it is possible to increase the mobility of carriersin the cladding layer and thereby lower the operating voltage of thedevice. From this perspective, it is preferable that the Al content inthe p-type cladding layer 308 be limited within the range from about0.06 to 0.09.

A preferred range of the thickness of the n-type AlGaN cladding layer303 is from 0.7 μm to 1.5 μm. This helps achieve a unimodal verticallateral mode, increased light confinement efficiency, enhanced opticalcharacteristics, and a lower threshold current density.

In the above description, an AlGaN ternary crystal was used as each ofthe cladding layers 303 and 308. Instead, a quaternary crystal such asAlInGaN, AlGaNP, or AlGaNAs may be used. For a reduced electricalresistance, the p-type cladding layer 308 may be given a superlatticestructure composed of a p-type AlGaN layer and a p-type GaN layer, or asuperlattice structure compose of a p-type AlGaN layer and a p-typeAlGaN layer, or a superlattice structure composed of a p-type AlGaNlayer and a p-type InGaN layer.

The description above deals with a case where the crystal was grown byusing a MOCVD machine. It is, however, also possible to use molecularbeam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE).

Next, a description will be given of the process of taking out theepitaxial wafer having the individual layers of the n-type nitridesemiconductor layer 104 formed on top of the n-type GaN substrate 101out of the MOCVD machine and then forming it into individual nitridesemiconductor laser device chips.

Producing Individual Devices

As the laser light waveguide region 105, a ridge-stripe portion isformed in the n-type GaN substrate 101, in the desired positiondescribed earlier with reference to FIG. 1. This is achieved byperforming etching from the obverse side of the epitaxial wafer to themiddle or lower end of the p-type cladding layer 308 so as to leave astripe-shaped portion. Here, the stripe was 1 to 3 μm wide, andpreferably 1.3 to 2 μm wide; the distance from the p-type guiding layer307 to the etching bottom surface was 0 to 0.1 μm. Thereafter, aninsulating film 310 was formed elsewhere than where the ridge-stripeportion was located. Here, the insulating film 310 was formed of AlGaN.The p-type GaN contacting layer 308 left unetched was exposed, andtherefore, on this portion and on the insulating film 310, a p-electrode311 was formed by vapor-depositing Pd/Mo/Au in this order.

The insulating film 310 may be formed, instead of the material mentionedabove, an oxide or nitride of silicon, titanium, zirconia, tantalum,aluminum, or the like. The combination of materials for the p-electrode311 may be, instead of the one mentioned above, Pd/Pt/Au, Pd/Au, orNi/Au.

Then, the reverse side (substrate side) of the epitaxial wafer waspolished to adjust its thickness to 80 to 200 μm so that the wafer wouldlater be easier to separate. On the reverse side of the substrate, ann-electrode 312 was formed in the order Hf/Al. The combination of thematerials for the n-electrode 312 may instead be Hf/Al/Mo/Au,Hf/Al/Pt/Au, Hf/Al/W/Au, Hf/Au, Hf/Mo/Au, or a modified version of anyof these where Hf is replaced with Ti or Zr.

Lastly, the epitaxial wafer was cleaved in the direction perpendicularto the ridge stripe direction to produce a Fabry-Perrot cavity with acavity length of 600 μm. A preferred range of the cavity length is from250 μm to 1,000 μm. Through this process, the wafer was cleaved intobars each having individual laser devices arranged laterally next to oneanother. In a nitride semiconductor laser device in which a stripe isformed along the<1-100> direction, the end surfaces of the cavity arethe {1-100} plane of the nitride semiconductor crystal. Cleaving wasachieved not by engraving scribe lines over the entire surface of thewafer by the use of a scriber, but by engraving scribe lines only inpart of the wafer, for example at opposite ends of the wafer or atplaces thereon corresponding to opposite ends of each chip, by the useof a scriber and then effecting cleaving by using those scribe lines asstarting points. Feedback may be achieved otherwise than by means of endsurfaces; for example, it may be achieved by means of diffractiongratings provided inside, a technique called DFB (distributed feedback),or by means of diffraction gratings provided outside, a technique calledDBR (distributed Bragg reflector).

After the formation of the cavity end surfaces of the Fabry-Perrotcavity, dielectric films of SiO₂ and TiO₂ having a reflectivity of about80% were alternately vapor-deposited on those end surfaces to formdielectric multiple-layer reflective films. The dielectricmultiple-layer reflective films may be formed of any other dielectricmaterial. Thereafter, each bar was separated into individual laserdevices, and, in this way, the semiconductor laser device 1 shown inFIG. 1 was obtained. The laser light waveguide region 105 (ridge stripe)was located in the middle of the laser chip, and the laser device 1 hada lateral width W of 400 μm.

In the n-type GaN substrate 101 used to produce the device structure,dislocation-concentrated regions 102 were arranged at a pith P of 400μm, high-luminescence regions 103 were arranged in the middle portionsof low-dislocation regions, and laser light waveguide regions 105 werearranged so that their distance from the high-luminescence regions 103was 80 μm and their distance t from the dislocation-concentrated regions102 was 120 μm. Accordingly, each semiconductor laser device (chip)includes one dislocation-concentrated region and one high-luminescenceregion. That is, in this embodiment, W=P and 2(t+d)=P.

Through the procedure described above, the chip of the nitridesemiconductor laser device 1 shown in FIGS. 1 and 3 was produced.

Characteristics of the Semiconductor Laser Device

The nitride semiconductor laser device 1 thus obtained had anarrowed-current portion located in an optimum position, achieving alife time of 5,000 hours or more under the following conditions: at alaser output power of 60 mW, and at an ambient temperature of 70° C.Incidentally, the inventors of the present invention also producedsemiconductor lasers according to the prior art described earlier andtested them under the same conditions to find that they had lives ofabout 1,000 hours.

Embodiment 2

The semiconductor laser device of Embodiment 2 of the invention issimilar to the semiconductor laser device 1 of Embodiment 1, and thedifference between them lies in that the inclination angle of thesurface of the n-type GaN substrate on which the device structure islaid is varied between 0° to 2°. The fabrication procedure here conformsto that for Embodiment 1. As described earlier in the “FabricationProcedure of the GaN Substrate” section, the substrate 101 may havedislocation-concentrated regions arranged in a pattern of stripes, ormay have dislocation-concentrated regions arranged in a pattern of dotsin a nested fashion. In a case where the dislocation-concentratedregions has a pattern of dots, they do not necessarily have to bearranged in a nested fashion. More specifically, so long as thesubstrate has dislocation-concentrated regions in a state clearlydistinguishable from low-dislocation regions and in addition, when seenfrom the direction perpendicular to the surface on which to grow anitride semiconductor layer, the dislocation-concentrated regions have asmaller area than the low-dislocation regions, it can be usedsatisfactorily irrespective of the arrangement of the individualdislocation-concentrated regions.

FIG. 5 shows the mean roughness of the epitaxial wafer surface asobserved when its inclination was varied between 0° to 2° in the <11-20>and <1-100> directions relative to the C plane. Irrespective of theinclination direction, with the inclination angle in the range from 0°to less than 0.30°, projections having the shape of hexagonal pyramidsdeveloped on the wafer surface, and their diameters were about 200 to400 μm. Because of these projections, the mean surface roughness asmeasured using a contact-probe-type step height tester at scanningintervals of 2 mm was 20 to 50 nm, which was ten or more times greaterthan the thickness of each of the well and barrier layers constitutingthe active layer, predicting degraded light emission characteristics ofthe active layer and a shorter life of the device.

With the inclination angle greater than 0.70°, the shape of theprojections that developed on the epitaxial wafer surface changed fromhexagonal pyramids, which were typically observed at small angles, tostripes with deep grooves, with the stripes, and thus also the grooves,lying at intervals of about 50 to 100 μm from one another. Since thegrooves were deep, the mean surface flatness was greater than 50 nm,resulting in a very rough surface. By contrast, with the inclinationangle in the range from 0.30° to 0.70°, no projections developed on theepitaxial surface, resulting in very satisfactory flatness. The meansurface flatness was about 10 nm, sufficiently small as compared withthe thickness of a well or barrier layer. This indicates that thequantum well active layer was produced uniformly in the wafer surface.

FIG. 6 shows the FWHM (full width at half maximum) of emitted lightplotted ageist the inclination angle, as observed when the same deviceas shown in FIG. 5 was energized to operate in the LED mode. Inaccordance with the surface morphology and the mean surface roughness,irrespective of the direction, light emission with a narrow FWHM wasobtained with the inclination angle in the range from 0.30° to 0.70°.This means that determining the inclination angle within the range from0.3° to 0.7° helps make the surface of the grown layer flat and formeach interface inside the quantum well active layer steeply anduniformly. These effects probably eliminated agglomeration of In insidethe active layer, resulting in sharp light emission with a narrow FWHM.

FIG. 7 shows the etch pit density (EPD) plotted against the inclinationangle as observed when the same device was subjected to etching in amixture of phosphoric acid and sulfuring acid at 250° for 20 minutesbefore the formation of electrodes and the density of etch pits thatappeared at the surface within the range of 200 μm of thedislocation-concentrated region was measured. In general, the pits thatappear when a nitride semiconductor is etched are classified into twogroups by their size, i.e., into small and large pits. It is consideredthat large pits correspond to blade-shaped dislocations and small pitsto spiral, blade-shaped, and all other dislocations. In the embodimentbeing described, small pits, which correspond to all dislocations, werecounted for evaluation. The figure shows that, in accordance with theenhancement of the surface flatness and of the light emissioncharacteristics, irrespective of the direction, with the inclinationangle in the range from 0.30° to 0.70°, the EPD dropped to levels of theorder of 10⁴ cm⁻². When the same measurements were made with a substratewith no surface inclination, within the range of 200 μm of thedislocation-concentrated region, the EPD was at levels of the order of10⁷ cm⁻². This indicates that, when the substrate is given no surfaceinclination, dislocations spread from the dislocation-concentratedregion into the grown layer.

Considering the area of a semiconductor laser device, in general, if theEPD is 10⁴ cm⁻³ or less, the number of dislocations included per deviceis one or less. Thus, giving the substrate a surface inclination isexpected to prolong device lives.

The device of this embodiment produced with the inclination angle in therange from 0.30° to 0.70° had a life time of 5,000 hour or more underthe following conditions: at a laser output power of 60 mW, and at anambient temperature of 70° C. Moreover, because of the depressionsresulting from the slower growth immediately above thedislocation-concentrated regions where the substrate surface wasterminated with nitrogen atoms, the grown layer was divided at the pitchbetween adjacent dislocation-concentrated regions. This helped reducethe residual stress in the device, and thus helped achieve a lower crackdensity than in the conventional device structure, leading to anincreased yield.

Embodiment 3

In Embodiment 3 of the invention, a GaN substrate that is given asurface inclination in the range from 0.30° to 0.70° as described inEmbodiment 1 is used, and crystal growth is performed with only thethickness of the n-type layers varied to see how the EPD and the meansurface roughness vary. The purpose is to grasp the thickness needed tomake the surface of the grown layer, grown on top of the substrate witha surface inclination, sufficiently flat to permit the quantum wellactive layer to be produced with satisfactory quality.

How crystal growth is performed will be described with reference to FIG.8. A GaN substrate 801 similar to that described in Embodiment 1 isused. As described earlier in the “Fabrication Procedure of the GaNSubstrate” section, the substrate 801 may have dislocation-concentratedregions arranged in a pattern of stripes, or may havedislocation-concentrated regions arranged in a pattern of dots in anested fashion. In a case where the dislocation-concentrated regions hasa pattern of dots, they do not necessarily have to be arranged in anested fashion. More specifically, so long as the substrate hasdislocation-concentrated regions in a state clearly distinguishable fromlow-dislocation regions and in addition, when seen from the directionperpendicular to the surface on which to grow a nitride semiconductorlayer, the dislocation-concentrated regions have a smaller area than thelow-dislocation regions, it can be used satisfactorily irrespective ofthe arrangement of the individual dislocation-concentrated regions.

As described earlier in connection with Embodiment 1, the substrate 801includes, as a part thereof, a dislocation-concentrated regions 802 and,as all the remaining part thereof, a low-dislocation region, and has, asa part of the low-dislocation region, a high-luminescence region 803parallel to the dislocation-concentrated regions 802. Using a MOCVDmachine, to NH₃ as a group V material and TMGa (trimethyl gallium) orTEGa (triethyl gallium) as a group III material, at a substratetemperature of 1,050° C., a GaN layer 804 was formed on top of the GaNsubstrate 801. Here, it is also possible to form first, before crystalgrowth at a high temperature about 1,000° C., a so-calledlow-temperature buffer layer at a comparatively low temperature of 500to 700° C. and then raise the temperature to form the GaN layer 804 at ahigh temperature about 1,000° C.

Through similar procedures, several samples in which the thickness ofthe GaN layer 804 was varied between 0.5 and 4 μm were produced, andfirst their mean surface roughness was measured using a step-heighttester. As shown in FIG. 9, with the thickness of the GaN layer 804 lessthan 1 μm, the roughness was 10 nm or more. For example, in a case wherea quantum well active layer is grown successively after the formation ofthe GaN layer, unless the GaN layer is 1 μm or more thick, the obtainedsurface flatness is expected to be insufficient, with no improvement inthe light emission characteristics.

Why the GaN layer needs to be 1 μm or more thick to obtain sufficientflatness is not definitely known. One possible explanation is asfollows. In the dislocation-concentrated region, the surface isterminated with nitrogen atoms. Thus, there, the materials are lessstable than elsewhere where the surface is terminated with gallium, andaccordingly the materials tend to be spewed out into where the surfaceis terminated with gallium. As a result, while the GaN layer is thin,the materials spewed out of the dislocation-concentrated region exhibitan uneven concentration distribution at the substrate surface. Thus,while the GaN layer is thin, surface irregularities are comparativelylikely to develop.

Not only with GaN, but also with any nitride semiconductor of thecomposition Al_(x)Ga_(y)InN (where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1),when it was grown, the grown layer needed to be 1 μm or more thick tosufficiently reduce the mean surface roughness and the EPD.

Likewise, also when multiple layers of any compositions fulfillingAl_(x)Ga_(y)In_(z)N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1) were laid,if the total thickness of the multiple layers was 1 μm or more,reductions was observed in the mean surface roughness and the EPD.Moreover, in all the cases described above, even when crystal growth wasperformed with an n- or p-type dopant added, the total thickness of thegrown layer needed to be 1 μm or more to obtain sufficient flatness andto achieve a sufficient EPD reduction to produce a high-quality multiplequantum well active layer.

Embodiment 4

In Embodiment 4 of the invention, using a GaN substrate similar to theone used in Embodiment 1, a p-type nitride semiconductor layer is grown,and its p-type carrier concentration is measured. This is to show howthe grown layer, as it is grown, is p-typified, without anypost-processing performed.

Through a procedure similar to that used in Embodiment 3, a 3 μm thickGaN layer was grown, and then, immediately after it was taken out of thegrowth machine, hole measurement was performed to evaluate theconcentration of p-type carriers. The thus measured p-type carrierconcentration was 5×10¹⁷ cm⁻³, indicating that the grown layer, as itwas grown, was p-typified. When the same sample was subjected toordinary heat treatment, the carrier concentration increased to 7×10¹⁷cm⁻³. The mechanism by which the grown layer, as it is grown, isp-typified is unknown. A possible explanation is that reduceddislocations inhibit the Mg taken into the crystal from binding tohydrogen, making passivation of the Mg difficult.

Not only with GaN, but also with any nitride semiconductor of thecomposition Al_(x)Ga_(y)In_(z)N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, andx+y+z=1), when it was grown, the grown layer, as it was grown, wasp-type, as was the case with GaN. As described earlier in the“Fabrication Procedure of the GaN Substrate” section, the substrate 101may have dislocation-concentrated regions arranged in a pattern ofstripes, or may have dislocation-concentrated regions arranged in apattern of dots in a nested fashion. In a case where thedislocation-concentrated regions has a pattern of dots, they do notnecessarily have to be arranged in a nested fashion. More specifically,so long as the substrate has dislocation-concentrated regions in a stateclearly distinguishable from low-dislocation regions and in addition,when seen from the direction perpendicular to the surface on which togrow a nitride semiconductor layer, the dislocation-concentrated regionshave a smaller area than the low-dislocation regions, it can be usedsatisfactorily irrespective of the arrangement of the individualdislocation-concentrated regions.

Embodiment 5

In Embodiment 5 of the invention, the nitride semiconductor laser device1 having a ridge-stripe structure described in connection withEmbodiment 1 is replaced with a nitride semiconductor laser device 2having a current blocking layer. Now, the nitride semiconductor laserdevice 2 of this embodiment having a current blocking layer will bedescribed with reference to FIG. 10.

In the semiconductor laser device 2 of this embodiment, on top of ann-type GaN substrate 1001, there are laid the following layers on oneanother in the order mentioned: an n-type GaN layer 1002, an n-typeIn_(0.07)Ga_(0.93)N crack prevention layer 1003, an n-typeAl_(0.1)Ga_(0.9)N cladding layer 1004, an n-type GaN light guiding layer1005, an active layer 1006, a p-type Al_(0.2)Ga_(0.8)N carrier blockinglayer 1007, a p-type GaN light guiding layer 1008, a p-typeAl_(0.1)Ga_(0.9)N first cladding layer 1009 a, a current blocking layer1010, a p-type Al_(0.1)Ga_(0.9)N second cladding layer 1009 b, a p-typeInGaN contacting layer 1011, a p-electrode 1012, and an n-electrode1013.

The current blocking layer 1010 is so laid as to block the currentinjected from the p-electrode 1012 in such a way that the current passesonly through a width secured between divided parts of the currentblocking layer 1010 as shown in FIG. 10. For example, the currentblocking layer 1010 may be a layer of Al_(0.25)Ga_(0.75)N. The currentblocking layer 1010 may have any. Al content other than 0.25. In thisembodiment, the opening in the current blocking layer 1010 serves as alaser light guiding region 1014, functioning like the one provided inEmbodiment 1.

As described earlier in the “Fabrication Procedure of the GaN Substrate”section, the substrate 1001 may have dislocation-concentrated regionsarranged in a pattern of stripes, or may have dislocation-concentratedregions arranged in a pattern of dots in a nested fashion. In a casewhere the dislocation-concentrated regions has a pattern of dots, theydo not necessarily have to be arranged in a nested fashion. Morespecifically, so long as the substrate has dislocation-concentratedregions in a state clearly distinguishable from low-dislocation regionsand in addition, when seen from the direction perpendicular to thesurface on which to grow a nitride semiconductor layer, thedislocation-concentrated regions have a smaller area than thelow-dislocation regions, it can be used satisfactorily irrespective ofthe arrangement of the individual dislocation-concentrated regions.

Embodiment 6

In Embodiment 6 of the invention, at least one element selected from thegroup of As, P, and Sb is added to the active layer of the nitridesemiconductor laser device 1 or 2. All the other features are as alreadydescribed.

In this embodiment, at least one element selected from the group of As,P, and Sb is added at least to the well layers included in the activelayer 305 or 1006 laid in the nitride semiconductor laser device 1 or 2.Here, let the total content of the elements belonging to theabove-mentioned group in the well layers be X, and let the content ofthe element N in the well layers be Y. Then, X is smaller than Y, and inaddition X/(X+Y) is equal to or smaller than 0.3 (30%), and preferablyequal to or smaller than 0.2 (20%). The lower limit of the total contentof the elements belonging to the above-mentioned group is 1×10¹⁸ cm⁻³ ormore.

As the content X exceeds 20%, concentration separation starts to occur,in which the contents of the individual elements vary from one region toanother within the well layers. As the content X exceeds 30%, theconcentration separation starts to shift to crystal system separation,in which a hexagonal crystal coexists with a cubic crystal, causing thecrystallinity of the well layers to start to degrade. On the other hand,as the total content of the elements of the above-mentioned group dropsbelow 1×10¹⁸ cm⁻³, it becomes difficult to obtain the benefits of addingthe above-mentioned elements to the well layers.

The benefits of this embodiment lie in that adding As, P, or Sb to thewell layers helps reduce the effective masses of electrons and holes inthe well layers and helps increase the mobility of electrons and holesin the well layers. In a semiconductor laser device, the former benefitmeans that a carrier inversion distribution for laser oscillation can beobtained through injection of less current, and the latter benefit meansthat, even when electrons and holes in the active layer disappear as aresult of their recombination for light emission, new electrons andholes are injected at a high rate by diffusion. That is, as comparedwith conventionally reported In GaN nitride semiconductor laser devicesthat contain none of the elements As, P, and Sb in their active layer,the nitride semiconductor laser device of this embodiment offers a lowthreshold current density and excellent noise characteristics.

As described earlier in the “Fabrication Procedure of the GaN Substrate”section, the substrate 1001 may have dislocation-concentrated regionsarranged in a pattern of stripes, or may have dislocation-concentratedregions arranged in a pattern of dots in a nested fashion. In a casewhere the dislocation-concentrated regions has a pattern of dots, theydo not necessarily have to be arranged in a nested fashion. Morespecifically, so long as the substrate has dislocation-concentratedregions in a state clearly distinguishable from low-dislocation regionsand in addition, when seen from the direction perpendicular to thesurface on which to grow a nitride semiconductor layer, thedislocation-concentrated regions have a smaller area than thelow-dislocation regions, it can be used satisfactorily irrespective ofthe arrangement of the individual dislocation-concentrated regions.

Embodiment 7

In Embodiment 7 of the invention, when the individual nitridesemiconductor layers are formed on top of the substrate, a selectivegrowth technique is used. In all the other respects, this embodiment isthe same as any of the thus far described embodiments.

A selective growth technique is one whereby a mask formed of a materialthat retards growth (for example, an oxide such as SiO₂ or a nitridesuch as SiN or AlN) and having openings formed therein is formed on thesubstrate beforehand so that, when the nitride semiconductor layers areformed on the substrate, growth proceeds in the lateral direction at anearly stage of growth. This helps effectively prevent cracks that maydevelop as the nitride semiconductor layers are grown. The mask may beformed so as to lay above the dislocation-concentrated region 102 andthe high-luminescence region 103, or independently of those regions. Itis preferable to lay the mask at least immediately below the laser lightwaveguide region 105, because it helps effectively prevent cracks thatmay develop in the laser light waveguide region 105.

Embodiments 1 to 7 thus far described all deal with cases where GaN isused as the substrate, but the substrate may be replaced with a nitridesemiconductor substrate formed of any of the materials mentioned earlierin the “SUMMARY OF THE INVENTION” section. The materials of the nitridesemiconductor layers grown on the nitride semiconductor substrate alsomay be replaced with any of the nitride semiconductor materialsmentioned earlier in the “SUMMARY OF THE INVENTION” section.

Embodiment 8

In Embodiment 8 of the invention, a nitride semiconductor laser deviceaccording to the invention is applied to a semiconductor opticalapparatus.

When a nitride semiconductor laser device according to the invention(with an oscillation wavelength of 330 to 550 nm) is used in asemiconductor optical apparatus such as an optical pickup apparatus, itoffers the following benefits. Any nitride semiconductor laser deviceaccording to the invention yields high output (30 mW), operates stablyeven in a high-temperature environment (60° C.), and offers a long life.This makes it suitable for use in a disc apparatus designed forhigh-density recording and reproduction, where high reliability issought (the shorter the wavelength, the higher the density with whichrecording and reproduction are possible).

FIG. 11 shows an outline of the configuration of, as an example of asemiconductor optical apparatus to which a nitride semiconductor laserdevice according to the invention is applied, an optical disc apparatus(an apparatus having an optical pickup, for example a DVD apparatus).The optical disc apparatus 1101 includes an optical pickup 1102, acontrol circuit 1103, a motor 1104 for rotating a disk D, and a motor1105 for moving the optical pickup 1102. The optical pickup 1102includes a semiconductor laser device 1106, a beam splitter 1107, amirror 1108, an objective lens 1109, and a photodetector 1110. Used asthe semiconductor laser device 1106 here is the nitride semiconductorlaser device 1 or 2 of any of the embodiments described thus far.

For recording of information, the laser light L emitted from thesemiconductor laser device 1106 is modulated by the control circuit 1103according to input information, then travels through the beam splitter1107, mirror 1108, and objective lens 1109, and then converges on therecording surface of the disk D, achieving the recording of informationto the disk D. Alternatively, while the semiconductor laser device 1106is emitting unmodulated laser light L, the magnetic field in the portionon the recording surface of the disk D on which the laser light Lconverges is modulated according to input information. This too achievesthe recording of information. For reproduction of information, laserlight L optically varied according to the pits arranged on the disk Dtravels through the objective lens 1109, mirror 1108, and beam splitter1107 to enter the photodetector 1110, by which the laser light L isconverted into a reproduced signal. The power of the laser light Loutputted from the semiconductor laser device 1106 is, for example,about 30 mW for recording and about 5 mW for reproduction.

Semiconductor laser devices according to the invention can be used, notonly in optical disc apparatuses having an optical pickup apparatus asthe one just described, but also in laser printers, bar code readers,projectors involving laser of three primary colors (blue, green, andred), and the like, and are thus suitable as high-output, long-lifelight sources.

Embodiment 9

In general, a substrate formed of a nitride semiconductor includes about5×10⁷ cm² of crystal defects at the substrate surface. By using a meansfor bending, eliminating, or otherwise treating such crystal defects, itis possible to obtain low-defect regions and thereby secure asufficiently long life in high-output power aging. Moreover, goodcrystallinity and uniform planer distribution on the substrate or in thecrystal layer grown on top of the substrate help enhance light emissionefficiency, reduce variations in characteristics, and increase yields.

FIG. 12 is a top view of the wafer as observed after the ridge-stripeportions 1204 of Embodiment 9 of the invention are formed thereon. FIG.13 is a sectional view of the GaN substrate of Embodiment 9. FIG. 14 isa sectional view of the laser diode of Embodiment 9.

In the GaN substrate, there exist defect-concentrated regions in apattern of stripes along the <1-100> direction, and, as the otherregions than those, there exist low-defect regions and parts where the{0001} facet is exposed. The top surface of the GaN substrate has anoff-angle relative to the C plane, and the off-angle is 0.70° in thedirection perpendicular to the stripes of the defect-concentratedregions (i.e., in the a1 direction) and is 0.3° in the directionparallel thereto (i.e., in the <1-100> direction).

Fabrication Procedure of the GaN Substrate

Now, the fabrication procedure of the n-type GaN substrate 1401 will bedescribed with reference to FIG. 15. FIG. 15(a) is a sectional view of aprincipal portion of a support base member 1501 having an n-type GaNlayer 1502 laid on top thereof. On the support base member 1501, then-type GaN layer 1502 is grown by HVPE (hydride vapor phase epitaxy) insuch a way that chiefly the {11-22} facet 1414 is exposed at the surfaceduring growth. This causes the surface to have a sawtooth-shaped sectionwith ridges and troughs. Near the tops of the ridges, however, isslightly exposed the {0001} plane 1503 in the shape of stripes.

Used as the support base member 1501 is a two-inch (111) GaAs wafer. Theridges and troughs have a periodic structure with a pitch P of 400 μm,and have a shape that extends, like ridges in a crop field, in the depthdirection of the figure. To determine where to locate these ridges andtroughs, a mask of SiO₂ or the like having opening corresponding to thetroughs may be formed beforehand on the substrate so that crystal growthproceeds with the facets kept exposed. That is, the openings of the maskare arranged in a pattern of stripes arranged parallel to the ≦1-100>direction of the GaN crystal with a pitch P of 400 μm. Thus, theopenings may have the shape of continuous stripes, or may be composed ofdot-shaped openings arranged in rows. The technique of (and theconditions for) maintaining crystal growth with the {11-22} facet keptexposed is disclosed in detail in Japanese Patent Application Laid-OpenNo. 2001-102307. The grown crystal was made n-type by being doped with Owhile growing.

By continuing the growth of the GaN crystal with this growth modemaintained, a 30 mm high ingot was produced on top of the support basemember 1501. FIG. 15(b) shows the ingot.

Using a slicer, this ingot was sliced into thin pieces (each an n-typeGaN substrate). The thin piece was then polished to produce aflat-surfaced n-type GaN substrate 1401 measuring two inches in diameterand 350 μm in thickness. The surface for epitaxial growth was specularlypolished. As shown in FIG. 13, the GaN substrate is so produced that thetop surface thereof has an off-angle of 0.7° in the directionperpendicular to and 0.30° in the direction parallel to the stripes ofthe defect-concentrated regions 1201.

Here, a preferred range of the off-angle in the perpendicular directionis from 0.2° to 2°, both ends inclusive, and a preferred range thereofin the parallel direction is 2° or less. The off-angle in the paralleldirection may be just zero (0°), but is preferably other than that,because it then makes uniform the width of the uniformly-light-emittingregions 1202. Moreover, it is preferable that the off-angle in theparallel direction be equal to or smaller than that in the perpendiculardirection.

FIG. 15(c) is a sectional view of a principal portion of the n-type GaNsubstrate 1401 thus produced, and FIG. 15(d) is a top view of aprincipal portion of the n-type GaN substrate 1401. This n-type GaNsubstrate 1401 was evaluated in the following manner.

When the surface of the n-type GaN substrate 1401 was closely inspectedunder a microscope, it was observed to be, though polished, notperfectly flat, but had small depressions in regions corresponding tothose where the bottommost parts of the troughs were located duringcrystal growth. Then, the sample was etched by being dipped in a mixtureof sulfuric acid and phosphoric acid heated to 250° C. so that etch pitsappeared at the surface. As a result, in regions corresponding to thosewhere the bottommost parts of the troughs were located during crystalgrowth, many etch pits appeared, attesting that those regions were wheredefects (or dislocations) were highly concentrated (i.e.,defect-concentrated regions 1201). That is, the depressions mentionedabove corresponded to these regions. As described above, the regions inwhich depressions were observed had highly concentrated dislocations,and thus were more easily eroded than other regions. This probably isthe reason that the depressions were formed there. Thedefect-concentrated regions were about 10 to 50 μm wide. All the otherregions were low-defect regions 1203 with EPDs (etch pit densities) ofthe order of 10² to 10⁵ cm⁻². The defect-concentrated regions 1201 wereobserved to have three or more orders of magnitude greater EPDs.

Moreover, with the sample irradiated with ultraviolet rays (the 365 nmspectral line of a Hg lamp can be used), the luminescence from thesurface was observed under a microscope (fluorescence microscopy). As aresult, in about the middle portions of the low-defect regions 1203sandwiched between the defect-concentrated regions 1201, stripe-shapedregions 1301 were observed that had comparatively clear boundaries andthat had different contrast from the adjoining regions. These regionsexhibit more intense light emission (luminescence), which can beobserved with the naked eye, than the adjoining regions, and areobserved to be yellowish and bright. These regions 1301 are the partswhich grew with the {0001} facet exposed during crystal growth. One ofthe probable reasons that these regions are observed to appear differentfrom the adjoining regions is that the dopant is absorbed differentlythere than in the adjoining regions. On the basis of the facts describedabove, in the present specification, these regions are called“high-luminescence regions.” During crystal growth, the parts that growwith the {0001} facet exposed do not always advance uniformly with aconstant width. Thus, the width of the high-luminescence regions 1301was about 0 to 30 μm, with slight fluctuations. It should be noted thatthe high-luminescence regions 1301 are omitted in FIGS. 12 and 14.

The formation of the GaN substrate 1401 may be achieved by anyvapor-phase crystal growth process other than HVPE; it may be formed,for example, by MOCVD (metalorganic chemical vapor deposition), MOVPE(metalorganic chloride vapor phase epitaxy), or sublimation.

Used as the support base member 1501 on which to grow the GaN substratemay be, instead of GaAs, a monocrystalline substrate that has sixfold orthreefold symmetry about an axis. That is, a monocrystal with hexagonalsymmetry or cubic symmetry can be used. With a cubic-symmetry crystal,using the (111) plane results in threefold symmetry. Examples ofhexagonal-symmetry crystals that can be used include sapphire, SiC,SiO₂, NdGaO₃, ZnO, GaN, AlN, and ZrB₂. Examples of cubic-symmetrycrystals of which a (111) plane substrate can be used include Si,spinel, MgO, and GaP. With these, GaN is grown on the C plane.

The mask that is used in the formation of the GaN substrate 1401 is laidby one of the following two methods. By one method, the mask is formeddirectly on the substrate. In this case, it is advisable to take apreparatory step prior to epitaxial growth as by depositing a GaN bufferlayer on the surface where the substrate will be exposed through theopenings in the mask. By the other method, a comparatively thin GaNlayer is formed beforehand on the substrate, and then the mask is formedon top. In many cases, the latter method proves to be preferable,permitting smooth progress of growth.

Epitaxial Growth of the Nitride Semiconductor Layer

Next, how a nitride semiconductor layer etc. are formed on top of then-type GaN substrate 1401 to fabricate a semiconductor laser device willbe described with reference to FIG. 14.

First, using a MOCVD machine, using NH₃ as a group V material and TMGa(trimethyl gallium) or TEGa (triethyl gallium) as a group III material,using SiH₄ as a dopant material, and using hydrogen or nitrogen as amaterial carrier gas, at a substrate temperature of 1,050°, a 3 μm thickn-type GaN layer 1402 was formed on the n-type GaN substrate 1401. Next,at a substrate temperature of 800° C., with TMIn (trimethyl indium) as agroup III material added to the already mentioned materials, a 40 nmthick n-type In_(0.07)Ga_(0.93)N crack prevention layer 1403 was formed.Next, the substrate temperature was raised to 1,050° C., and, using TMAl(trimethyl aluminum) or TEAl (triethyl aluminum) as a group IIImaterial, a 1.2 μm thick n-type Al_(0.1)Ga_(0.9)N cladding layer 1404was formed. The dopant material was so prepared as to contain, as then-type impurity used in these steps, 5×10¹⁷/cm³ to 1×10¹⁹/cm³ of Si.Subsequently, a 0.1 μm thick n-type GaN light guiding layer 1405 (with aSi impurity concentration of 1×10¹⁶/cm³ to 1×10¹⁸/cm³) was formed.

Thereafter, the substrate temperature was lowered to 750° C., and anactive layer (multiple quantum well structure) 1406 composed of 4 nmthick In_(0.1)Ga_(0.9)N well layers and 8 nm thick In_(0.01)Ga_(0.99)Nbarrier layers that are laid alternately on one another in three periodsin the following order: a barrier layer, a well layer, a barrier layer,a well layer, a barrier layer, a well layer, and then a barrier layer.Meanwhile, SiH₄ (with a Si impurity concentration of 10¹⁶ to 10¹⁸/cm³)was introduced into the barrier layers or to both the barrier and welllayers. It is preferable to suspend the growth for a period of onesecond or more but less than 180 seconds between adjacent barrier andwell layers or between adjacent well and barrier layers, because thishelps enhance the flatness of the individual layers and helps reduce theFWHM (full width at half maximum) of emitted light. A preferred materialused to add As to the active layer is AsH₃ (arsine), TBAs (tertiarybutyl arisine), or TMAs (trimethyl arisine); a preferred material usedto add P to the active layer is PH₃ (phosphine), TBP (tertiary butylphosphine), or TMP (trimethyl phosphine); a preferred material used toadd Sb to the active layer is TMSb (trimethyl antimony) or TESb(triethyl antimony). Used as the N material in the formation of theactive layer may be, instead of NH₃, a hidrazine material such asdimethyl hydrazine, or azide material such as ethyl azide.

In a case where the active layer includes In_(x)Ga_(1-x)N quantum wellsformed in a plurality of layers, or in a case where As or P is added tothe active layer to form a quantum well active layer, it is known that,if there are threading dislocations in the quantum wells, In segregatesat the dislocations. Accordingly, in a case where the active layerincludes quantum wells containing In_(x)Ga_(1-x)N as main componentelements, it is essential to minimize dislocations (defects) to obtainsatisfactory laser characteristics.

Next, the substrate temperature was raised back to 1,050° C., and a 20nm thick p-type Al_(0.3)Ga_(0.7)N carrier blocking layer 1407, a 0.1 μmthick p-type GaN light guiding layer 1408, a 0.5 μm p-typeAl_(0.1)Ga_(0.9)N cladding layer 1409, and a 0.1 μm thick p-type GaNcontacting layer 1410 were formed in this order. As the p-type impurityused in these steps, EtCp₂Mg (bisethylcyclopentadienyl magnesium) wasused as a material, and was added so that the Mg concentration was 10¹⁸to 2×10²⁰ cm⁻³. As the Mg material here, it is also possible to useanother cyclopenta Mg material such as cyclopentadienyl magnesium orbismethylcyclopentadienyl magnesium. It is preferable that the p-typeimpurity concentration in the p-type GaN contacting layer 1410 increasetoward a p-electrode 1412. This helps reduce the contact resistance dueto the formation of the p-electrode. Moreover, to eliminate the residualhydrogen in the p-type layers which impedes the activation of Mg used asthe p-type impurity, a trace amount of oxygen may be mixed during thegrowth of the p-type layers.

After the p-type GaN contacting layer 1410 was grown in this way, allthe contents inside the reactor of the MOCVD machine were replaced withnitrogen carrier gas and NH₃, and the temperature was lowered at a rateof 60° C. per minute. When the substrate temperature became 800° C., thesupply of NH₃ was stopped, and then, after this substrate temperaturewas maintained for five minutes, it was lowered further to roomtemperature. Here, a preferred range of temperature at which thesubstrate is maintained is from 650° C. to 900° C., and a preferredrange of duration for which that temperature is maintained is from threeto ten minutes. A preferred rate at which the temperature is lowered is30° C. or more per minute. The grown film thus produced was evaluated byRaman measurement, with the following results. Thanks to the proceduredescribed above, after the wafer was taken out of the MOCVD machine,even without p-typifying annealing, the grown film exhibited p-typeproperties (the activation of Mg) immediately after growth. In addition,the contact resistance due to the formation of the p-electrode was low.Combining the above technique with conventional p-typifying annealingproved to be preferable, because it led to enhanced Mg activation.

The n-type In_(0.07)Ga_(0.93)N crack prevention layer 1403 may have anIn content other than 0.07, or the entire InGaN crack prevention layermay be omitted. To prevent cracks, instead of Si, Ge may be used as ann-type impurity.

The active layer 1406 starts with a barrier layer and ends with abarrier layer; instead, it may start with a well layer and end with awell layer. The number of well layers formed may be any other than threeas specifically described above; so long as the number is ten or less, alow threshold current density permitted continuous wave operation atroom temperature. Two or more but six or less layers were particularlypreferable, because of a low threshold current density. The active layermay further contain Al.

In the active layer 1406, a necessary amount of Si was added to both thewell and barrier layers. This impurity does not necessarily have to beadded, but adding an impurity such as Si to the active layer resulted inmore intense light emission. The impurity may be, other than Si, atleast one selected from the group of impurities consisting of O, C, Ge,Zn, and Mg. A preferred total amount of impurities added is about 10¹⁷to 8×10¹⁸/cm³. The impurity may be added to, instead of both the welland barrier layers, the well layers alone or the barrier layers alone.

The p-type Al_(0.3)Ga_(0.7)N carrier blocking layer 1407 may have acomposition other than this specific composition. Using AlGaN having Inadded thereto is preferable, because it becomes p-type even when grownat a lower temperature, and thus helps alleviate the damage inflicted onthe active layer during crystal growth. The entire carrier blockinglayer may be omitted, but providing it leads to a lower thresholdcurrent density. This is because the carrier blocking layer serves toconfine carriers inside the active layer. Increasing the Al content inthe carrier blocking layer is preferable, because it permits carriers tobe confined more powerfully. By contrast, reducing the Al content downto a level that barely maintains confinement of carriers is alsopreferable, because it increases the carrier mobility inside the carrierblocking layer and thereby reduces the electrical resistance.

Here, an Al_(0.1)Ga_(0.9)N crystal was used as each of the p-typecladding layer 1409 and the n-type cladding layer 1404. Instead, anAlGaN ternary crystal with an Al content other than 0.1 may be used. Asthe Al content in the mixed crystal increases, the differences in energygap and in refractive index from the active layer increase. This permitsmore efficient confinement of carriers and light in the active layer,leading to a lower threshold current density. By contrast, reducing theAl content down to a level that barely maintains the confinement ofcarriers and light, it is possible to increase the mobility of carriersin the cladding layer and thereby lower the operating voltage of thedevice.

A preferred range of the thickness of the n-type AlGaN cladding layer1404 is from 0.7 μm to 1.5 μm. This helps achieve a unimodal verticallateral mode, increased light confinement efficiency, enhanced laseroptical characteristics, and a lower threshold current density.

In the above description, an AlGaN ternary crystal was used as each ofthe p-type cladding layer 1409 and the n-type cladding layer 1404.Instead, a quaternary crystal such as AlInGaN, AlGaNP, or AlGaNAs may beused. For a reduced electrical resistance, the p-type cladding layer1409 may be given a superlattice structure composed of a p-type AlGaNlayer and a p-type GaN layer, or a superlattice structure composed of ap-type AlGaN layer and a p-type InGaN layer.

The description above deals with a case where the crystal was grown byusing a MOCVD machine. It is, however, also possible to use molecularbeam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE).

The subsequent process is one for taking out the epitaxial wafer havingthe individual layers of the nitride semiconductor layer formed on topof the n-type GaN substrate 1401 as described above out of the MOCVDmachine and then forming it into individual nitride semiconductor laserdevice chips. Here, in FIG. 14, the p-type cladding layer 1409 has anelevated portion, which is the shape formed through the processdescribed later. In Embodiment 9, on the surface of the epitaxial waferon completion of the formation of nitride semiconductor laser devices,in the regions consisting of low-defect regions 1203 sandwiched betweendefect-concentrated regions 1201 and parts 1301 that had grown with the{0001} facet 1503 kept exposed, uniformly-light-emitting regions 1202were obtained that extended within a width of about 120 μm of thelow-defect regions 1201 in the a1 direction (in FIG. 14, the rightwarddirection). The thickness of these uniformly-light-emitting regions 1202was about 200 Å smaller than that of the other parts of the low-defectregions 1203. Why the uniformly-light-emitting regions 1202 are obtainedis not definitely known, but they are obtained where there arestripe-shaped defect-concentrated regions 1201 and in addition there isan off-angle in the direction perpendicular to the stripes of thedefect-concentrated regions 1201. By further producing an appropriateoff-angle also in the direction parallel to the defect-concentratedregions 1201, it is possible to make substantially constant the width ofthe uniformly-light-emitting regions 1202.

FIG. 16 is a sectional view of a principal portion of the n-type GaNsubstrate 1401 having an off-angle relative to the C plane, showing therelationship between the top surface of the GaN substrate 1401 and theC-axis direction as observed in the direction perpendicular to thestripes of the defect-concentrated regions 1201. Of the two directionsperpendicular to the stripes of the defect-concentrated regions 1201,the one in which the C axis forms, in the direction perpendicularthereto, an acute angle a to the surface of the GaN substrate is the a1direction. In FIG. 16, the a1 direction is indicated as the rightwarddirection.

The uniformly-light-emitting regions 1202 are obtained in the a1direction with respect to the defect-concentrated regions 1201.Moreover, the shape of the uniformly-light-emitting regions 1202 dependson the direction of the off-angle. This is because the magnitude of theoff-angle in the a1 direction affects the direction, width, and depth inand with which the uniformly-light-emitting regions 1202 are obtained,and the magnitude of the off-angle in the parallel direction affects thewidth of the uniformly-light-emitting regions 1202.

The tendency is that, the greater the off-angle in the direction (a1direction) perpendicular to the defect-concentrated regions 1201, thenarrower and deeper the uniformly-light-emitting regions 1202, andtherefore that, the gentler the inclination, the wider and shallower theuniformly-light-emitting regions 1202. The off-angle in the directionparallel to the defect-concentrated regions 1201 makes substantiallyconstant the width of the uniformly-light-emitting regions 1202.However, there is a tendency that, the shaper the inclination in theparallel direction, the smaller the width of theuniformly-light-emitting regions 1202. If this off-angle in the paralleldirection is just or nearly zero, the width of theuniformly-light-emitting regions 1202 is not constant but tends to varygreatly between about 50 μm to 250 μm. These facts indicate thatincreasing the off-angle both in the perpendicular and paralleldirections makes it impossible to obtain satisfactoryuniformly-light-emitting regions. Thus, it is preferable that theoff-angle in the direction perpendicular to the stripe-shapeddefect-concentrated regions 1201 be equal to or greater than 0.2° butequal to or smaller than 2°, and that the off-angle in the paralleldirection be equal to or smaller than 2°. Moreover, to secure asufficient and substantially uniform width to produce ridge-stripeportions 1204 or narrowed-current portions on theuniformly-light-emitting regions 1202, it is preferable that theoff-angle in the parallel direction be smaller than that in theperpendicular direction.

Moreover, in the regions within about 30 μm of the edges of thedefect-concentrated regions 1201, there remain strains in the shape ofthe defect-concentrated regions 1201 and strains nearby, producingportions elevated as compared with the uniformly-light-emitting regions1202. On the other hand, right above the defect-concentrated regions1201, where epitaxial growth of a nitride semiconductor is moredifficult, depressions develop. The nitride semiconductor laser devicesthus produced were, just as the GaN substrate was, inspected under afluorescence microscope, with the following result. Except in regionswhere the high-luminescence regions and the defect-concentrated regions1201 had poor morphology, the uniformly-light-emitting regions 1202emitted light substantially over the entire surface thereof.

Producing Individual Devices

Ridge-stripe portions for confining light in a direction parallel to thesubstrate were formed in positions 70 μm away from thedefect-concentrated regions 1201 on the uniformly-light-emitting regions1202 of the epitaxial wafer described above. In a case where a substratehaving high-luminescence regions as described above is used, it ispreferable that ridge-stripe portions be formed somewhere else than inthose regions. This is because the high-luminescence regions have alower dopant content or a lower degree of dopant activation and thus hashigher resistivity than other regions, producing a distribution in thecurrent that is injected into the laser device. Even on the surface ofthe uniformly-light-emitting regions 1202, in parts thereof near thedefect-concentrated regions 1201, the surface of the epitaxial wafer iselevated. Thus, it is preferable to form ridge-stripe portions 1204 inpositions 40 μm or more away from the defect-concentrated regions 1201.

The ridge-stripe portions 1204 are formed by performing etching from thewafer to the middle of the p-type cladding layer 1409 so as to leavestripe-shaped portions. Here, the stripes are 1 to 3 μm wide, andpreferably 1.3 to 2 μm wide; the distance from the p-type guiding layer1408 to the etching bottom surface was 0.1 to 0.4 μm. Thereafter, aninsulating film 1411 was formed elsewhere than where the ridge-stripeportions 1204 were located. Here, the insulating film 1411 was formed ofAlGaN. The p-type GaN contacting layer 1410 left unetched was exposed,and therefore, on this portion and on the insulating film 1411, ap-electrode 1412 was formed by vapor-depositing Pd/Mo/Au in this order.

The insulating film 1411 may be formed, instead of the materialmentioned above, an oxide or nitride of silicon, titanium, zirconia,tantalum, aluminum, or the like. The combination of materials for thep-electrode may be, instead of the one mentioned above, Pd/Pt/Au, Pd/Au,Ni/Au, or the like.

Then, the reverse side (substrate side) of the epitaxial wafer waspolished to adjust its thickness to 80 to 200 μm so that the wafer wouldlater be easier to separate.

On the reverse side of the substrate, an n-electrode 1413 was formed inthe order Hf/Al. The combination of the materials for the n-electrodemay instead be Hf/Al/Mo/Au, Hf/Al/Pt/Au, Hf/Al/W/Au, Hf/Au, Hf/Mo/Au, ora modified version of any of these where Hf is replaced with Ti or Zr.

Lastly, the above epitaxial wafer is cleaved in the directionperpendicular to the ridge stripe direction to produce Fabry-Perrotcavities with a cavity length of 500 μm. A preferred range of the cavitylength is from 300 μm to 1,000 μm. Through this process, the wafer iscleaved into bars each having individual laser devices arrangedlaterally next to one another. In nitride semiconductor laser devices inwhich the direction of ridge stripes is formed along the <1-100>direction, the end surfaces of the cavity are the {1-100} plane of thenitride semiconductor crystal. Cleaving is achieved not by engravingscribe lines over the entire surface of the wafer by the use of ascriber, but by engraving scribe lines only in part of the wafer, forexample at opposite ends of the wafer, by the use of a scriber and theneffecting cleaving by using those scribe lines as starting points.Feedback in the laser cavity may be achieved otherwise than in themanner described above; for example, it may be achieved by a well-knowntechnique such as DFB (distributed feedback) or DBR (distributed Braggreflector).

After the formation of the cavity end surfaces of the Fabry-Perrotcavities, dielectric films of SiO₂ and TiO₂ having a reflectivity ofabout 70% are alternately vapor-deposited on those end surfaces to formdielectric multiple-layer reflective films. The dielectric materials mayinstead be SiO₂/Al₂O₃. Thereafter, each bar is separated into individuallaser devices, and, in this way, the semiconductor laser device shown inFIG. 3 is obtained. The laser light waveguide region (ridge stripe) waslocated on the uniformly-light-emitting region, and the laser device hada lateral width W (the width of each laser after being separated fromthe bar) of 400 μm. On the original n-type GaN substrate 1401, there arearranged defect-concentrated regions 1201 at a pitch P of 400 μm. Thedefect-concentrated regions 1201 are not needed on laser chips, andtherefore the separation may be effected so that those regions areexcluded. In this case, it is preferable that cleaving be performed sothat the cleavage planes be 10 μm or more away from the laser lightguide regions (ridge stripes), and it is further preferable thatcleaving be performed so that the elevated regions near thedefect-concentrated regions 1201 be excluded. In this way, the nitridesemiconductor laser chip shown in FIG. 14 is produced.

Characteristics of the Semiconductor Laser Device

The characteristics of the above nitride semiconductor laser device weremeasured, with the following result. The life time of 5,000 hours ormore was achieved under the following conditions: at a laser output of60 mW, and at an ambient temperature of 70° C.

Embodiment 10

FIG. 17 is a sectional view of the GaN substrate of Embodiment 10 of theinvention. FIG. 18 is a top view of the wafer after the formation of theridge stripe portions 1204 of Embodiment 10. FIG. 19 is a sectional viewof the laser diode of Embodiment 10. In FIG. 17, the a1 direction is theleftward direction.

In Embodiment 10, the top surface of the GaN substrate havingdefect-concentrated regions arranged with a pitch of 500 μm in the<1-100> direction is the C plane, and has an off-angle of 1.1° in thedirection (the a1 direction) perpendicular to the stripes of thedefect-concentrated regions and 0.5° in the direction (the <1-100>direction) parallel thereto. In all the other respects, the structurehere is the same as in Embodiment 9.

As in Embodiment 9, on top of the GaN substrate 1801 are formed, at asubstrate temperature of 1,050° C., the following layers on one anotherin the order named: a 3 μm thick n-type GaN layer 1402, a 1 μm thickn-type Al_(0.1)Ga_(0.9)N cladding layer 1403, and a 0.1 μm thick n-typeGaN light guiding layer 1404. Subsequently, at 750° C., an active layer(multiple quantum well structure) 1406 composed of 4 nm thickIn_(0.1)Ga_(0.9)N well layers and 8 nm thick In_(0.01)Ga_(0.99)N barrierlayers that are laid alternately on one another in five periods in thefollowing order: a barrier layer, a well layer, a barrier layer, a welllayer, a barrier layer, a well layer, and then a barrier layer. Next,the substrate temperature is raised back to 1,050° C., and a 20 nm thickp-type Al_(0.3)Ga_(0.7)N carrier blocking layer 1407, a 0.1 μm thickp-type GaN light guiding layer 1408, a 0.5 μm p-type Al_(0.1)Ga_(0.9)Ncladding layer 1409, and a 0.1 μm thick p-type GaN contacting layer 1410are formed in this order. On the top surface of the nitridesemiconductor laser devices grown in this way, as shown in FIG. 18,uniformly-light-emitting regions 1202 were obtained with a width of 80μm in the a1 direction with respect to the defect-concentrated regions1201.

Subsequently, ridge-stripe portions 1204 were formed above theuniformly-light-emitting regions 1202, 60 μm away from the edges of thedefect-concentrated regions 1201. Then, with the lateral width W of eachlaser device determined to be 400 μm so as not to include thedefect-concentrated regions 1201, separation was performed so as toobtain the designed cavity length (600 μm). In the separation process,it is preferable that the cleavage planes be 10 μm or more away from theridge-stripe portions, and it is further preferable that the elevatedregions near the defect-concentrated regions 1201 be excluded from thechips. In Embodiment 10, separation was effected at positions 40 μm and440 μm away from the defect-concentrated regions 1201 in the a1direction, so that the produced chips each had a width W of 400 μm.

Embodiment 11

FIG. 20 is a sectional view of the GaN substrate of Embodiment 11 of theinvention, and FIG. 21 is a top view of the wafer after the formation ofthe ridge-stripe portions 1204 of Embodiment 11.

In Embodiment 11, the off-angle is 0.2° in the direction (the a1direction) perpendicular to the stripes of the defect-concentratedregions 1201 and just zero (0°) in the direction parallel thereto. Inall the other respects, the structure here is the same as in Embodiment9.

The width of the obtained uniformly-light-emitting regions 1202fluctuated between about 80 μm to 200 μm in the a1 direction withrespect to the defect-concentrated regions 1201. It is when theoff-angle in the parallel direction is just or nearly zero that thewidth of the uniformly-light-emitting regions 1202 fluctuates.Ridge-stripes are formed above the uniformly-light-emitting regions1202, 60 μm away from the edges of the defect-concentrated regions 1201.

Embodiment 12

In Embodiment 12, the off-angle is 20 in the direction (the a1direction) perpendicular to the stripes of the defect-concentratedregions 1201 and 2° in the direction (the <1-100> direction) parallelthereto. In all the other respects, the structure here is the same as inEmbodiment 9.

The obtained uniformly-light-emitting regions 1202 had a width of about50 μm in the perpendicular direction (the a1 direction) from the edgesof the defect-concentrated regions 1201 with respect to the stripes ofthe defect-concentrated regions 1201. Ridge-stripe portions 1204 areformed above the uniformly-light-emitting regions 1202, 40 μm away fromthe edges of the defect-concentrated regions 1201.

In Embodiments 10 to 12, the same device characteristics as inEmbodiment 9 were obtained.

1. A nitride semiconductor laser device comprising a nitridesemiconductor substrate and a plurality of nitride semiconductor layerslaid on top thereof, wherein the nitride semiconductor layers include anactive layer having a quantum well structure by being composed of one ormore well layers and one or more barrier layers, and an acceptor dopinglayer, wherein the nitride semiconductor substrate includes, as a partthereof, a dislocation-concentrated region and, as all the remainingpart thereof, a low-dislocation region, and wherein the nitridesemiconductor layers laid immediately above the dislocation-concentratedregion and the low-dislocation region have a depression immediatelyabove the dislocation-concentrated region.
 2. The nitride semiconductorlaser device according to claim 1, wherein, irrespective of shapes ofthe dislocation-concentrated region and the low-dislocation regionexposed at a top surface of the nitride semiconductor as seen from abovethe top surface, the dislocation-concentrated region has a smaller areathan the low-dislocation region.
 3. The nitride semiconductor laserdevice according to claim 1, wherein the nitride semiconductor substratehas a hexagonal crystal, and wherein the dislocation-concentrated regionand the low-dislocation region respectively are a c-plane and a C plane.4. The nitride semiconductor laser device according to claim 1, whereina topmost surface of the nitride semiconductor substrate is slanted atan angle in a range from 0.30° to 0.70° relative to a C plane.
 5. Thenitride semiconductor laser device according to claim 4, wherein adirection in which a plane orientation of the top surface of the nitridesemiconductor substrate is slanted relative to the C plane is a <11-20>or <1-100> direction.
 6. The nitride semiconductor laser deviceaccording to claim 1, wherein the acceptor doping layer as is grownreadily exhibits p-type conductivity.
 7. The nitride semiconductor laserdevice according to claim 1, wherein a hole concentration in theacceptor doping layer is 10¹⁷ cm⁻³ or more.
 8. The nitride semiconductorlaser device according to claim 1, wherein a mean surface roughness of abottom surface of the active layer in the quantum well structure issmaller than a thickness of the active layer.
 9. The nitridesemiconductor laser device according to claim 1, wherein a totalthickness from the nitride semiconductor substrate through a layerimmediately below the active layer in the nitride semiconductor layersis 1 μm or more.
 10. The nitride semiconductor laser device according toclaim 1, wherein the active layer contains at least one element selectedfrom the group consisting of As, P, and Sb.
 11. A semiconductor opticalapparatus comprising as a light source the nitride semiconductor laserdevice according to claim
 1. 12. A method of fabricating a nitridesemiconductor laser device, comprising the step of: forming, on top of anitride semiconductor substrate including, as a part thereof, adislocation-concentrated region and, as all the remaining part thereof,a low-dislocation region, a plurality of nitride semiconductor layersincluding an active layer having a quantum well structure by beingcomposed of one or more well layers and one or more barrier layers andan acceptor doping layer, wherein the nitride semiconductor layers laidimmediately above the dislocation-concentrated region and thelow-dislocation region have a depression immediately above thedislocation-concentrated region.
 13. The method of fabricating a nitridesemiconductor laser device according to claim 12, wherein a topmostsurface of the nitride semiconductor substrate is slanted at an angle ina range from 0.30° to 0.70° relative to a C plane.
 14. The method offabricating a nitride semiconductor laser device according to claim 12,wherein the acceptor doping layer as is grown readily exhibits p-typeconductivity.
 15. The method of fabricating a nitride semiconductorlaser device according to claim 12, wherein a hole concentration in theacceptor doping layer is 10¹⁷ cm⁻³ or more.
 16. A nitride semiconductorlight-emitting device having a plurality of nitride semiconductor layerson top of a nitride semiconductor substrate, wherein the nitridesemiconductor substrate includes, as a part thereof, a stripe-shapeddefect-concentrated region in which crystal defects concentrate and, asall the remaining part thereof, a low-defect region, and wherein a topsurface of the nitride semiconductor substrate has an off-angle in adirection perpendicular to a direction of the stripe of thedefect-concentrated region.
 17. The nitride semiconductor light-emittingdevice according to claim 16, wherein the off-angle is in a range from0.2° to 2.0°, both ends inclusive.
 18. The nitride semiconductorlight-emitting device according to claim 16, wherein the top surface ofthe nitride semiconductor substrate has an off-angle in a directionparallel to the direction of the stripe of the defect-concentratedregion, and wherein the off-angle is 2° or less.
 19. The nitridesemiconductor light-emitting device according to claim 16, wherein adepression-like uniformly-light-emitting region that emits light withlittle unevenness is formed on a top surface of the nitridesemiconductor substrate, and wherein a ridge-stripe portion or anarrowed-current portion is formed on a top surface of theuniformly-light-emitting region.